Implement support for mixed sampled render targets

src/gpu/gl/GrGLGpu.h

Index: src/gpu/gl/GrGLGpu.h
diff --git a/src/gpu/gl/GrGLGpu.h b/src/gpu/gl/GrGLGpu.h
index c9e280c3cbc8b7478c8d9c25e24b326086df42b8..4f185cb11086304a7d22e6a2bb545284baf521fb 100644
--- a/src/gpu/gl/GrGLGpu.h
+++ b/src/gpu/gl/GrGLGpu.h
@@ -448,6 +448,7 @@ private:
     } fHWBlendState;
 
     TriState fMSAAEnabled;
+    TriState fCoverageModulationEnabled;
 
     GrStencilSettings           fHWStencilSettings;
     TriState                    fHWStencilTestEnabled;