png_loader(static): Fix the colorspace of an image with an alpha channel

Set colorspace to ABGR when colortype of lodepng is LCT_RGBA.
Since an image without an alpha channel becomes an ARGB colorspace with LCT_RGB,
it is the same as the default colorspace.

src/loaders/png/tvgPngLoader.cpp

@@ -213,5 +213,7 @@ void PngLoader::run(unsigned tid)
 
     lodepng_decode(&image, &width, &height, &state, data, size);
 
+    if (state.info_png.color.colortype == LCT_RGBA) colorSpace = SwCanvas::ABGR8888;
+
     _premultiply((uint32_t*)(image), width, height);
 }