Add comment about optimization

src/PixelArray.cc

@@ -114,6 +114,9 @@ PixelArray::PixelArray(Canvas *canvas, int sx, int sy, int width, int height):
       uint8_t g = *pixel >> 8;
       uint8_t b = *pixel;
       dst[bx + 3] = a;
+
+      // Performance optimization: fully transparent/opaque pixels
+      // can be processed more efficiently
       if (a != 0 && a != 255) {
         float alpha = (float) a / 255;
         dst[bx + 0] = (int)((float) r / alpha);