Feature/DXT

DXT1/DXT5/RGBA8 DDS support. Normal maps can be packed into DXT5nm now.
Notes:
- DXT textures must be pre-flipped vertically, because UNPACK_FLIP_Y_WEBGL doesn't work;
- IE doesn't support mip-mapped DXT = doesn't support DXT;
- Not EVERY texture looks good after DXT;
- DXT is not supported by mobiles.

Why use DXT:
- less VRAM usage;
- faster texture lookup (reading less memory on lookups).

Why use RGBA8 DDS:
- it's basically raw pixel data. IE/iOS Webkit are bad at preserving PNG precision, so it might be better to load simple non-processed RGBA8 there.