Update to NVTT 2.1.1

[leper]

NVTT 2.1.0 was (finally) released some time last year. Actually fixing the build is a single line addition, porting the patches (at least those that seem useful) is a bit more work.

TODO: Actually use 2.1.1.

It would also be nice to possibly unbundle NVTT, since there aren't any patches that are actually needed (memory leak fixes, compression fixes, etc) anymore.

​https://github.com/leper/0ad/tree/nvtt-2.1.0 imports the new release, removes patches merged upstream, fixes the build, fixes some warnings, and ports a few patches (not all though).

This seems to work on Linux (but that ignores distributing binaries or other machines than mine really), though I must admit that I didn't convert many textures since I did this.

Testing on Windows and OSX and BSD and other Linux distros would be needed (at least the first of those needs someone to actually build libraries). Also someone porting the other patches would be nice (if you want to do that in my branch ask for access).

[leper]

2.1.1 was released last week.