PNGLoader: Add load_png_from_memory method

Author: https://github.com/deoxxa
Commit: bfde6acd8c
Pull-request: https://github.com/SerenityOS/serenity/pull/565
Reviewed-by: https://github.com/awesomekling

Libraries/LibDraw/PNGLoader.h

@@ -3,3 +3,4 @@
 #include <LibDraw/GraphicsBitmap.h>
 
 RefPtr<GraphicsBitmap> load_png(const StringView& path);
+RefPtr<GraphicsBitmap> load_png_from_memory(const u8*, size_t);