ETC2 crunched support

[K0lb3]

Atm decrunch doesn't support crunched versions of ETC2.

Addition to decrunch.pyx Format:
ETC1 = 10
ETC2 = 11
ETC2A = 12
ETC1S = 13
ETC2AS = 14

The /crunch code from Unity-Technologies/crunch results in a build error.

[K0lb3]

I replaced some parts of the old crn_decomp.h with parts of the new one from Unity-Technology and also replaced the other 2 files with those from Unity-Tech.

The result looks fine,
it can decode crunched ETC2 now.

crunch.zip

It also works in concert with Unitypack/Adds ability to decompress ETC and ETC2 images pull request.

[jleclanche]

Implemented in 6a21030