Faster SetImage for rpi-rgb-led-matrix (applied by first_time_install.sh at build
time; the submodule itself stays at its pinned commit).

Copying a frame into the panel buffer was the biggest CPU cost LEDMatrix owns on
large panels: the binding's SetPixelsPillow walked the image column by column
and called SetPixel per pixel, and each SetPixel read-modify-writes one word per
PWM bit plane, 2KB apart, so consecutive pixels were a whole double-row apart and
almost every write missed the cache. This patch:

 * FrameCanvas gets its own SetPixelsPillow: row by row, one bulk SetPixels call
   per row;
 * Framebuffer::SetPixels clips once, looks colours up once per pixel, walks each
   row's designators in order and writes the bit planes branch-free;
 * the base Canvas.SetPixelsPillow loop (RGBMatrix.SetImage) is row-major.

The bit-plane buffer is byte-identical to the old code's (882 memcmp checks over
noise/gradient/solid/low-value/sparse images, clipped offsets, pwm 7/8/11,
brightness 1/50/90/100, inverse colours, luminance correction off and a pixel
mapper). Measured on a Pi 4 at 512x64: 6.0-6.3 ms -> 1.8 ms per frame through
the Python binding; on hdpi (Pi 4, 4x128x64) frame copy 6.57 -> 2.21 ms and the
display process 139% -> 103% of a core.

LEDMatrix always draws into the canvas that is not on screen and swaps it in
(DisplayManager.update_display), so the write order cannot show as tearing.

Against hzeller/rpi-rgb-led-matrix 1ee4f76.

diff --git a/bindings/python/rgbmatrix/core.pyx b/bindings/python/rgbmatrix/core.pyx
index 230d87f..babc3bb 100644
--- a/bindings/python/rgbmatrix/core.pyx
+++ b/bindings/python/rgbmatrix/core.pyx
@@ -2,6 +2,7 @@
 
 from libcpp cimport bool
 from libc.stdint cimport uint8_t, uint32_t, uintptr_t
+from libc.stdlib cimport malloc, free
 import cython
 
 cdef extern from "Python.h":
@@ -59,8 +60,9 @@ cdef class Canvas:
 
         buffer = get_pillow_buffer(image_capsule)
 
-        for col in range(max(0, -xstart), min(width, frame_width - xstart)):
-            for row in range(max(0, -ystart), min(height, frame_height - ystart)):
+        # Row-major: walks both the image and the bitplane buffer sequentially.
+        for row in range(max(0, -ystart), min(height, frame_height - ystart)):
+            for col in range(max(0, -xstart), min(width, frame_width - xstart)):
                 pixel = buffer[row][col]
                 r = (pixel ) & 0xFF
                 g = (pixel >> 8) & 0xFF
@@ -86,6 +88,41 @@ cdef class FrameCanvas(Canvas):
     def SetPixel(self, int x, int y, uint8_t red, uint8_t green, uint8_t blue):
         (<cppinc.FrameCanvas*>self._getCanvas()).SetPixel(x, y, red, green, blue)
 
+    @cython.boundscheck(False)
+    @cython.wraparound(False)
+    def SetPixelsPillow(self, int xstart, int ystart, int width, int height, object image_capsule):
+        # Same result as Canvas.SetPixelsPillow(), but hands each image row
+        # to the C++ bulk FrameCanvas::SetPixels() instead of calling the
+        # virtual SetPixel() once per pixel.
+        cdef cppinc.FrameCanvas* my_canvas = <cppinc.FrameCanvas*>self._getCanvas()
+        cdef int col_start = max(0, -xstart)
+        cdef int col_end = min(width, my_canvas.width() - xstart)
+        cdef int row_start = max(0, -ystart)
+        cdef int row_end = min(height, my_canvas.height() - ystart)
+        cdef int row, col, pixel
+        cdef int *src
+        cdef cppinc.Color *line
+        cdef int **buffer
+
+        if col_end <= col_start or row_end <= row_start:
+            return
+        buffer = get_pillow_buffer(image_capsule)
+        line = <cppinc.Color*>malloc((col_end - col_start) * sizeof(cppinc.Color))
+        if line == NULL:
+            raise MemoryError()
+        try:
+            for row in range(row_start, row_end):
+                src = buffer[row]
+                for col in range(col_start, col_end):
+                    pixel = src[col]
+                    line[col - col_start].r = pixel & 0xFF
+                    line[col - col_start].g = (pixel >> 8) & 0xFF
+                    line[col - col_start].b = (pixel >> 16) & 0xFF
+                my_canvas.SetPixels(xstart + col_start, ystart + row,
+                                    col_end - col_start, 1, line)
+        finally:
+            free(line)
+
 
     property width:
         def __get__(self): return (<cppinc.FrameCanvas*>self._getCanvas()).width()
diff --git a/bindings/python/rgbmatrix/cppinc.pxd b/bindings/python/rgbmatrix/cppinc.pxd
index 8bec241..314332d 100644
--- a/bindings/python/rgbmatrix/cppinc.pxd
+++ b/bindings/python/rgbmatrix/cppinc.pxd
@@ -25,6 +25,7 @@ cdef extern from "led-matrix.h" namespace "rgb_matrix":
         FrameCanvas *SwapOnVSync(FrameCanvas*, uint8_t)
 
     cdef cppclass FrameCanvas(Canvas):
+        void SetPixels(int, int, int, int, Color*) nogil
         bool SetPWMBits(uint8_t)
         uint8_t pwmbits()
         void SetBrightness(uint8_t)
diff --git a/lib/framebuffer.cc b/lib/framebuffer.cc
index 36d138b..aee62ca 100644
--- a/lib/framebuffer.cc
+++ b/lib/framebuffer.cc
@@ -807,11 +807,60 @@ void Framebuffer::SetPixel(int x, int y, uint8_t r, uint8_t g, uint8_t b) {
   }
 }
 
+// Bulk version of SetPixel(); produces exactly the same bitplane content.
+// Faster because it hoists the per-pixel work out of the loop: the color
+// mapping becomes one 256-entry table built per call (each channel maps
+// independently through the same function), the pixel designators of a row
+// are contiguous in the PixelDesignatorMap, and the bit-plane loop is
+// branchless (the color bits are effectively random, so the branches in
+// SetPixel() mispredict a lot).
 void Framebuffer::SetPixels(int x, int y, int width, int height, Color *colors) {
-  for (int iy = 0; iy < height; ++iy) {
-    for (int ix = 0; ix < width; ++ix) {
-      SetPixel(x + ix, y + iy, colors->r, colors->g, colors->b);
-      ++colors;
+  PixelDesignatorMap *const mapper = *shared_mapper_;
+  const int ix_start = std::max(0, -x);
+  const int ix_end = std::min(width, mapper->width() - x);
+  const int iy_start = std::max(0, -y);
+  const int iy_end = std::min(height, mapper->height() - y);
+  if (ix_start >= ix_end || iy_start >= iy_end) return;
+
+  // Common case (luminance correction, no inversion): use the precomputed
+  // table directly; otherwise build one. Cheap enough to do per call, which
+  // matters for callers that send one row at a time.
+  uint16_t local_map[256];
+  const uint16_t *color_map;
+  if (do_luminance_correct_ && !inverse_color_) {
+    color_map = ColorLookupTable::GetLookup(brightness_).color;
+  } else {
+    for (int c = 0; c < 256; ++c) {
+      uint16_t unused1, unused2;
+      MapColors(c, 0, 0, &local_map[c], &unused1, &unused2);
+    }
+    color_map = local_map;
+  }
+
+  const int min_bit_plane = kBitPlanes - pwm_bits_;
+  gpio_bits_t *const plane_start = bitplane_buffer_ + columns_ * min_bit_plane;
+  for (int iy = iy_start; iy < iy_end; ++iy) {
+    const Color *c = colors + iy * width + ix_start;
+    const PixelDesignator *designator = mapper->get(x + ix_start, y + iy);
+    for (int ix = ix_start; ix < ix_end; ++ix, ++c, ++designator) {
+      const long pos = designator->gpio_word;
+      if (pos < 0) continue;  // non-used pixel marker.
+      const uint16_t red = color_map[c->r];
+      const uint16_t green = color_map[c->g];
+      const uint16_t blue = color_map[c->b];
+      const gpio_bits_t r_bits = designator->r_bit;
+      const gpio_bits_t g_bits = designator->g_bit;
+      const gpio_bits_t b_bits = designator->b_bit;
+      const gpio_bits_t designator_mask = designator->mask;
+      gpio_bits_t *bits = plane_start + pos;
+      for (int plane = min_bit_plane; plane < kBitPlanes; ++plane) {
+        const gpio_bits_t color_bits =
+          (r_bits & -(gpio_bits_t)((red >> plane) & 1))
+          | (g_bits & -(gpio_bits_t)((green >> plane) & 1))
+          | (b_bits & -(gpio_bits_t)((blue >> plane) & 1));
+        *bits = (*bits & designator_mask) | color_bits;
+        bits += columns_;
+      }
     }
   }
 }