perf(install): build rpi-rgb-led-matrix with a faster SetImage (#736)

first_time_install.sh applies patches/rpi-rgb-led-matrix/0001-bulk-setimage.patch just before building the Python binding and reverts it afterwards (and from the EXIT trap), so the submodule stays at its pinned commit. The patch copies each image row with one bulk FrameCanvas::SetPixels call and writes the bit planes branch-free: on a 512x64 Pi 4 the frame copy went from 6.57 ms to 2.21 ms. A patch that no longer applies is reported and skipped. Existing installs get it on a rebuild (RPI_RGB_FORCE_REBUILD=1).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

first_time_install.sh

@@ -232,6 +232,51 @@ _sync_rgb_submodule() {
     fi
     return 0
 }
+
+# LEDMatrix's own changes to the library live in patches/rpi-rgb-led-matrix/ and
+# are applied only for the build: _apply_rgb_patches before it, _revert_rgb_patches
+# after it, success or not. The checkout is left exactly as it was, so `git pull`
+# and _sync_rgb_submodule never meet local modifications in the submodule.
+# A patch that no longer applies (a submodule bump, a hand-edited checkout) is
+# reported and skipped -- the unpatched library still builds and works, so it is
+# never fatal. One that is already applied is left alone and not reverted.
+_RGB_APPLIED_PATCHES=()
+
+_apply_rgb_patches() {
+    local sub="$PROJECT_ROOT_DIR/rpi-rgb-led-matrix-master"
+    local dir="$PROJECT_ROOT_DIR/patches/rpi-rgb-led-matrix" patch name
+    _RGB_APPLIED_PATCHES=()
+    [ -d "$dir" ] || return 0
+    for patch in "$dir"/*.patch; do
+        [ -f "$patch" ] || continue
+        name=$(basename "$patch")
+        if _git_as_repo_owner -C "$sub" apply --check "$patch" >/dev/null 2>&1; then
+            if _git_as_repo_owner -C "$sub" apply "$patch"; then
+                _RGB_APPLIED_PATCHES+=("$patch")
+                echo "Applied library patch $name"
+            else
+                echo "⚠ Could not apply library patch $name; building without it"
+            fi
+        elif _git_as_repo_owner -C "$sub" apply --reverse --check "$patch" >/dev/null 2>&1; then
+            echo "Library patch $name is already applied"
+        else
+            echo "⚠ Library patch $name does not apply to this checkout; building without it"
+        fi
+    done
+    return 0
+}
+
+_revert_rgb_patches() {
+    local sub="$PROJECT_ROOT_DIR/rpi-rgb-led-matrix-master" i
+    # Last applied first, in case two patches touch the same file.
+    for ((i = ${#_RGB_APPLIED_PATCHES[@]} - 1; i >= 0; i--)); do
+        if ! _git_as_repo_owner -C "$sub" apply --reverse "${_RGB_APPLIED_PATCHES[i]}"; then
+            echo "⚠ Could not revert $(basename "${_RGB_APPLIED_PATCHES[i]}"); restore the checkout with: git -C $sub checkout -- ."
+        fi
+    done
+    _RGB_APPLIED_PATCHES=()
+    return 0
+}
 # --- end rpi-rgb-led-matrix checkout helpers ---------------------------------
 
 # Determine the Project Root Directory (where this script is located)
@@ -342,10 +387,12 @@ else
     lm_remove_build_swap() { return 0; }
 fi
 
-# Remove the temporary build swapfile no matter how the script ends. Step 6
-# tears it down itself; this is the backstop for the error path, since
-# on_error ends in `exit` and EXIT traps still run.
-trap 'lm_remove_build_swap' EXIT
+# Remove the temporary build swapfile, and take any library patches back out
+# of the submodule, no matter how the script ends. Step 6 does both itself;
+# this is the backstop for the error path (on_error ends in `exit` and EXIT
+# traps still run) and for an interrupted build. _revert_rgb_patches only
+# touches patches it applied, so running it twice is harmless.
+trap 'lm_remove_build_swap; _revert_rgb_patches' EXIT
 
 # Helpers
 retry() {
@@ -1289,9 +1336,11 @@ else
         fi
         BUILD_OUTPUT=$(mktemp)
         BUILD_SUCCESS=false
+        _apply_rgb_patches
         if run_rgbmatrix_build "$BUILD_JOBS" "$BUILD_OUTPUT"; then
             BUILD_SUCCESS=true
         fi
+        _revert_rgb_patches
         cat "$BUILD_OUTPUT" >> "$LOG_FILE"
         if [ "$BUILD_SUCCESS" != true ]; then
             print_rgbmatrix_build_failure "$BUILD_OUTPUT"