mesa: Add ARB_ES3_compatibility flag.

mesa: Add ARB_ES3_compatibility flag.

Adding this now makes it easier to develop and test GLES3 features, since we
can do initial development and testing using desktop GL.  Later GLSL compiler
patches check for either ctx->Extensions.ARB_ES3_compatibility or
_mesa_is_gles3 to allow certain features (i.e., "#version 300 es").

[v2, idr]: Just edits to the commit message.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Carl Worth <cworth@cworth.org>