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

This allows any 2Bpp/4Bpp Surfaces to be written via saveThumbnail
and loadThumbnail now. It furthermore will preserve the loaded Surface
in loadThumbnail.
Merge pull request #256 from lordhoto/thumbnail-pixelformat

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