mesa: Add blend barrier entry point and driver hook.

Both MESA_shader_framebuffer_fetch_non_coherent and the non-coherent
variant of KHR_blend_equation_advanced will use this driver hook to
request coherency between framebuffer reads and writes.  This
intentionally doesn't hook up glBlendBarrierMESA to the dispatch layer
since the extension isn't exposed to applications yet, see [1]
for more details.

[1] https://lists.freedesktop.org/archives/mesa-dev/2016-July/124028.html

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/main/barrier.c

@@ -108,3 +108,17 @@ _mesa_MemoryBarrierByRegion(GLbitfield barriers)
       ctx->Driver.MemoryBarrier(ctx, barriers);
    }
 }
+
+void GLAPIENTRY
+_mesa_BlendBarrierMESA(void)
+{
+   GET_CURRENT_CONTEXT(ctx);
+
+   if (!ctx->Extensions.MESA_shader_framebuffer_fetch_non_coherent) {
+      _mesa_error(ctx, GL_INVALID_OPERATION,
+                  "glBlendBarrier(not supported)");
+      return;
+   }
+
+   ctx->Driver.BlendBarrier(ctx);
+}

src/mesa/main/barrier.h

@@ -47,4 +47,7 @@ _mesa_MemoryBarrier(GLbitfield barriers);
 void GLAPIENTRY
 _mesa_MemoryBarrierByRegion(GLbitfield barriers);
 
+void GLAPIENTRY
+_mesa_BlendBarrierMESA(void);
+
 #endif /* BARRIER_H */

src/mesa/main/dd.h

@@ -957,6 +957,18 @@ struct dd_function_table {
    void (*MemoryBarrier)(struct gl_context *ctx, GLbitfield barriers);
    /** @} */
 
+   /**
+    * GL_MESA_shader_framebuffer_fetch_non_coherent rendering barrier.
+    *
+    * On return from this function any framebuffer contents written by
+    * previous draw commands are guaranteed to be visible from subsequent
+    * fragment shader invocations using the
+    * MESA_shader_framebuffer_fetch_non_coherent interface.
+    */
+   /** @{ */
+   void (*BlendBarrier)(struct gl_context *ctx);
+   /** @} */
+
    /**
     * \name GL_ARB_compute_shader interface
     */