GRAPHICS: Serialize PixelFormat for thumbnails and allow 4Bpp surfaces.

This allows any 2Bpp/4Bpp Surfaces to be serialized via saveThumbnail and loadThumbnail now. It furthermore will preserve the loaded Surface in loadThumbnail.

Another possible solution would be what pull request #245 is doing, which would be requring that 4Bpp thumbnails are ARGB8888. This pull request goes a bit further, since it would allow abitrary non 1-Bpp Surfaces to be serialized.