Some new features and bugfixes

- Documented YCG3/YCG4
- Added support for YCG1/YCG3 compression for all tools that is capable
to compress DXT5
- Tested YCoCg Scaled output from ATI Compressor/NVDxtLib, with no good
result but test code remained
- ETC2 compression now works
- cleanups in codecs
- now texture average color can be stored in unused sections of DDS
header. Used for Global Illumination in darkplaces.
- OmniLib is used to load quake sprite files.
- new option "-zipcompression" which allows to change zip compression
mode (0 for no compression, 9 for best compression).