Support for BC7 texture compression

Morrowind's low-resolution textures suffer tremendously from DXT1 compression artifacts. Aside from being necessary for OpenMW to load textures for Skyrim SE/Fallout 4/Fallout 76 in the future, BC7 compression would greatly benefit Morrowind textures as well as any future mods, as it has the same 4:1 compression ratio as DXT3/DXT5 but almost none of the artifacting that plagues those formats.

A showcase of the various compression formats compared to the uncompressed texture: https://imgsli.com/MTE2OTcy/