spirv: add support for SPV_KHR_post_depth_coverage

Allow the capability to be exposed, and convert the new execution mode
into fs state.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/shader_info.h

@@ -59,6 +59,7 @@ struct spirv_supported_capabilities {
    bool stencil_export;
    bool atomic_storage;
    bool storage_8bit;
+   bool post_depth_coverage;
 };
 
 typedef struct shader_info {