mesa: Add a copy of the Khronos gl.xml (SVN #31705).
mesa: Add a copy of the Khronos gl.xml (SVN #31705).

The intention here is to keep a pristine copy of the upstream gl.xml that
can be updated at any time with a new version, and use that to generate
Mesa code from instead of our private XML.

Reviewed-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler