Fixed bugs with sRGB handling on swizzled textures

- there are two types of sRGB swizzled texture now. 1 internal color
sRGB, texture is linear (engine should decode in shader). 2. Unswizzling
does not affect sRGB (currently there are no such formats).
- Swizzling code moved to mipmap generation. This slows down the
compression process, bug fixes alot of errors on a combination of
different tools/swizzled formats.
- YCG3, YCG4 formats removed, replaced by sRGB YCG1/YCG2
- Fixed crash on PVRTC compression
- Preliminary support for PVRTC2 compression (current version of PvrTex
lib crashes when compressing PVRTC files).
- New superior pixel scaling technique - xbrz and sBRz (which is 4x xBRz
then backscale to 2x usign nearest neightbor - gives sharper edges).
- RGB -> BGR swapping now done just before uploading texture to
compression tool, this fixes various bugs with different tool/format
combinations
Scale2X