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/shader_info.h

@@ -116,6 +116,7 @@ typedef struct shader_info {
           * ARB_shader_image_load_store.
           */
          bool early_fragment_tests;
+         bool post_depth_coverage;
 
          /** gl_FragDepth layout for ARB_conservative_depth. */
          enum gl_frag_depth_layout depth_layout;