mesa: Document reasons for allowing XFB drawing modes in GLES 3.1 w/GL_OES_geometry_shader
mesa: Document reasons for allowing XFB drawing modes in GLES 3.1 w/GL_OES_geometry_shader

Originally this patch added the checks to allow the draw calls with XFB,
but commit 2dabd497 beat me to it.

Signed-off-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>
Reviewed-by: Ilia Mirkin <bcbbb7aa705ec3a5fc5824c01a845186c3d62fca@alum.mit.edu>
Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler