mesa: Expose GL_NV_ES1_1_compatibility

@ajax alternative to !18216 (closed) that'd also work for me, even though the extension doesn't have an upstream spec (yet). I suspect this probably isn't acceptable upstream like this, but if it'd be acceptable after such a spec is published, this is probably what I'd want to keep downstream - though I'd really prefer not to have to keep downstream changes if I could avoid it.