Add dds bc7 support to blender

Currently blender can not open or view dds files saved in its newest bc7 format. This is not a brand new thing as the bc7 came out some years ago (2010?). Having bc7 (and bc6?) support is important for game development. I'd very much want to see my bc7 textures inside blender but for now I have to export my textures in two formats. Bc7 for the game and png for blender which is pretty annoying.