We haven't had a proper implementation for COMPRESS_PVRTC2 (which is PVRTC1 2-bpp) in years,
so let's drop it instead of keeping a compress type which doesn't work.
The other enum values were renamed to clarify that our PVRTC2 and PVRTC4 are respectively
PVRTC1 2-bpp and PVRTC1 4-bpp. PVRTC2 2-bpp and 4-bpp are not implemented yet.