i965: Add i965 plumbing for ARB_post_depth_coverage for i965 (gen9+).

This extension allows the fragment shader to control whether values in
gl_SampleMaskIn[] reflect the coverage after application of the early
depth and stencil tests.

Signed-off-by: Plamena Manolova <plamena.manolova@intel.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

docs/relnotes/13.1.0.html

@@ -44,6 +44,7 @@ Note: some of the new features are only available with certain drivers.
 </p>
 
 <ul>
+<li>GL_ARB_post_depth_coverage on i965/gen9+</li>
 <li>GL_NV_image_formats on any driver supporting GL_ARB_shader_image_load_store (i965, nvc0, radeonsi, softpipe)</li>
 </ul>