mesa: Add GL and GLSL 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>

src/compiler/glsl/ast.h

@@ -605,6 +605,11 @@ struct ast_type_qualifier {
          /** \{ */
          unsigned blend_support:1; /**< Are there any blend_support_ qualifiers */
          /** \} */
+
+         /**
+          * Flag set if GL_ARB_post_depth_coverage layout qualifier is used.
+          */
+         unsigned post_depth_coverage:1;
       }
       /** \brief Set of flags, accessed by name. */
       q;