GrGLInterface: Add support for NV_framebuffer_mixed_samples

src/gpu/gl/GrGLInterface.cpp

Index: src/gpu/gl/GrGLInterface.cpp
diff --git a/src/gpu/gl/GrGLInterface.cpp b/src/gpu/gl/GrGLInterface.cpp
index dba005976f366251f2c7e8177a67fb55303d1654..2e3dd1f039b5bb178eb420680bb2bf6da9610327 100644
--- a/src/gpu/gl/GrGLInterface.cpp
+++ b/src/gpu/gl/GrGLInterface.cpp
@@ -505,5 +505,11 @@ bool GrGLInterface::validate() const {
         }
     }
 
+    if (fExtensions.has("GL_NV_framebuffer_mixed_samples")) {
+        if (NULL == fFunctions.fCoverageModulation) {
+            RETURN_FALSE_INTERFACE
+        }
+    }
+
     return true;
 }