added support for glMinSampleShading

src/gpu/GrPrimitiveProcessor.h

Index: src/gpu/GrPrimitiveProcessor.h
diff --git a/src/gpu/GrPrimitiveProcessor.h b/src/gpu/GrPrimitiveProcessor.h
index e8bb449888460c33dce1cbeb07f5cf7e570eeb15..3300ec757d86359993c243c4e84569284f024c0f 100644
--- a/src/gpu/GrPrimitiveProcessor.h
+++ b/src/gpu/GrPrimitiveProcessor.h
@@ -217,6 +217,10 @@ public:
      */
     virtual const char* getDestColorOverride() const { return nullptr; }
     
+    virtual float getSampleShading() const {
+        return 0.0;
+    }
+
 protected:
     GrPrimitiveProcessor() : fVertexStride(0) {}