GrGLInterface: Add support for NV_framebuffer_mixed_samples

src/gpu/gl/GrGLInterface.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #include "gl/GrGLInterface.h"
 #include "gl/GrGLExtensions.h"
@@ skipping 487 matching lines @@
                 NULL == fFunctions.fStencilThenCoverStrokePath ||
                 NULL == fFunctions.fStencilThenCoverFillPathInstanced ||
                 NULL == fFunctions.fStencilThenCoverStrokePathInstanced ||
                 NULL == fFunctions.fProgramPathFragmentInputGen ||
                 NULL == fFunctions.fPathMemoryGlyphIndexArray) {
                 RETURN_FALSE_INTERFACE
             }
         }
     }
 
+    if (fExtensions.has("GL_NV_framebuffer_mixed_samples")) {
+        if (NULL == fFunctions.fCoverageModulation) {
+            RETURN_FALSE_INTERFACE
+        }
+    }
+
     return true;
 }