Add support for bc7.

[tristanjl]

Adds bc7 decompression and opengl support for bc7.

BC7 decoding function from here - https://github.com/richgel999/bc7enc - available under MIT or public domain licences

[AzP]

Sorry to comment on this specific bug, but I don't know where else to do it.

It seems DevIL has fallen rather dormant again... Any chance someone will pick up a fork to merge the 21 pull requests that are sitting around?