Allow alpha in _reflect textures

So it could be used in custom GLSL shader
ETC2 texture compression

Implemented ETC2 compressed textures (require ARB_ES3_compatibility).
Loader uses DDS format with new fourCC codes added: ETC1 (ETC2 is
backwards compatible with ETC1), ETC2 (ETC2 RGB), ETCP (ETC2 with
punchthrough alpha), ETCA (ETC2 RGBA with separately encoded alpha).
sRGB is supported via new 0x40000000 DDFP_ flag (not a part of standart,
but used by some other tools like NVidia Texture Tools). Since not all
hardware supports ETC1/ETC2 textures, before loading generic dds/ file,
loader first checks etc1/ and etc2/ folders. New cvars
r_texture_dds_load_etc1, r_texture_dds_load_etc2 control this behavior.
sRGB flag also affects DXT1-DXT5 textures.