Mesa 21.0 is now working with Haiku for software OpenGL rendering

A number of patches worked on for Haiku OS back for Mesa 20.x were freshened up and with some extra tweaking and code cleaning those patches have now been merged for Mesa 21.0. This includes factoring out a lot of the OpenGL legacy dispatch code and a lot of cleanups around the Softpipe driver handling.

With Mesa 21.0-devel as of today, it's at least enough where Mesa Git can now be built on Haiku OS and yield working OpenGL rendering with the LLVMpipe software.

Neat, and a testament to Haiku being in a far better state than many people seem to think.