GL: Add R8B8_R8G8 / B8R8_G8R8 formats, tweak G8B8.

Based on how the first two swizzle, most likely G8B8 works that way too.
GL: Add Y16_X16_FLOAT tex format (untested.)
GL: Add Y16_X16 tex format (untested.)
Typo.
Merge pull request #241 from unknownbrackets/texformats

Add a few more texformats, fix cellPngDec