[Feature Request] BC7 formatted DDS file support (DXGI header+BPTC)

[SirYodaJedi]

Source ports such as ioQuake3 and FTEQW support BC7-compressed textures (ARB_texture_compression_bptc), which is significantly higher quality than classic DXTn/S3TC compression, while having the same vram footprint as DXT5/BC3. Radiant and Q3Map2 currently only support DXTn formats (with code that hasn't been touched much since 2003); it'd be nice to be able to use BC7 textures directly without needing to maintain an uncompressed TGA/PNG next to it for editor/compiler use. This would require new code to parse the extended "DX10" header, or to call an image library that supports DX10 DDS files.