Add patching support for GRAY8 images.
Add patching support for GRAY8 images.

... or rather, "just let libpng do everything". Who knows when these ever become patch-relevant, but oh well.

And that's it! All ANM graphic formats done!

thpatch/thcrap,VBChunguk/thcrap,thpatch/thcrap,thpatch/thcrap,VBChunguk/thcrap,thpatch/thcrap,VBChunguk/thcrap,thpatch/thcrap