Avoid updating glPathStencilFunc unless it changes

src/gpu/GrStencil.h

Index: src/gpu/GrStencil.h
diff --git a/src/gpu/GrStencil.h b/src/gpu/GrStencil.h
index d94a45fad2bef6e6cc0a7416c511b68b74fc05f4..28042b0b2eb95ef6ad900c1bc0f6c03f3e3aa0a8 100644
--- a/src/gpu/GrStencil.h
+++ b/src/gpu/GrStencil.h
@@ -277,10 +277,14 @@ public:
 
     void invalidate()  {
         // write an illegal value to the first member
-        fPassOps[0] = (GrStencilOp)(uint8_t)-1;
+        fPassOps[0] = kStencilOpCount;
         fFlags = 0;
     }
 
+    bool isValid() const {
+        return fPassOps[0] < kStencilOpCount;
+    }
+
     bool operator == (const GrStencilSettings& s) const {
         static const size_t gCompareSize = sizeof(GrStencilSettings) -
                                            sizeof(fFlags);