Optimize getImageData()

Mostly for fully transparent pixels

src/PixelArray.cc

@@ -114,10 +114,16 @@ 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;
-      float alpha = (float) a / 255;
-      dst[bx + 0] = (int)((float) r / alpha);
-      dst[bx + 1] = (int)((float) g / alpha);
-      dst[bx + 2] = (int)((float) b / alpha);
+      if (a != 0 && a != 255) {
+          float alpha = (float) a / 255;
+          dst[bx + 0] = (int)((float) r / alpha);
+          dst[bx + 1] = (int)((float) g / alpha);
+          dst[bx + 2] = (int)((float) b / alpha);
+      } else {
+          dst[bx + 0] = r;
+          dst[bx + 1] = g;
+          dst[bx + 2] = b;
+      }
     }
     dst += dstStride;
   }