bc7 support from dhewm3

[y2keeth]

would be a cool addition
https://github.com/dhewm/dhewm3/releases/tag/1.5.5_RC3
example
https://www.moddb.com/mods/dhewm3-hidef2k

[y2keeth]

or maybe rbdoom3bfg's bimage tho I cannot get 2k or higher textures to work in rbdoom3bfg.
RobertBeckebans/RBDOOM-3-BFG#1082
or another idea improved rxgb support for textures not just locals
but bc7 is what a lot of modern games use

[themuffinator]

wouldn't hurt to implement this, i'll add this to the list!

[themuffinator]

implemented as follows:
c05b732
de238e7

please open an issue if there are problems with it