glsl: Allow compatibility shaders with MESA_GL_VERSION_OVERRIDE=...

glsl: Allow compatibility shaders with MESA_GL_VERSION_OVERRIDE=...

Previously if you used MESA_GL_VERSION_OVERRIDE=3.3COMPAT, Mesa exposed
an OpenGL 3.3 compatibility profile context (with various unimplemented
features and bugs), but still refused to compile shaders with

   #version 330 compatibility

This patch simply adds a small bit of plumbing to let that through.

Of course the same caveats apply: compatibility profile is still not
supported (and will not be supported), so there are no guarantees that
anything will work.

Tested-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>