Implemented sRGB detection for PNG/JPEG + DDS loader cleanups

sRGB RGBA textures now can be detected for PNG and JPEG formats by
checking embedded ICC profile. Loader supported profiles: sRGB
IEC61966-2.1/sRGB IEC61966-2-1/sRGB IEC61966/* wsRGB.
This behavior is optional and controlled by non-saveable cvars:
r_texture_png_iccmode, r_texture_jpeg_iccmode.
ETC2 DDS loader part modified to support per-texture extension (as
proposed for pre-ES3 hardware). Rewritten code of custom DDS folders
(etc1/etc2) for easier adding of new compressed formats.