Haiku: Major refactoring build fixes
This is the result of a lot of hard work by @X547 on Mesa 20.x I've done some tweaking and some cleanup.
- Dispatch code: A lot of the "really legacy" OpenGL dispatch code has been refactored away, with some improved debugging on OpenGL locking issues.
- softpipe: Lots of cleanups of hacks I did long ago to get it working
Sorry for the size, we were pretty behind master with these.