Add "sample locations" feature to GrProcessor

src/gpu/GrGpu.h

Index: src/gpu/GrGpu.h
diff --git a/src/gpu/GrGpu.h b/src/gpu/GrGpu.h
index 5d35fcf859bca3792681e9e6ff5037e92fc9fdf0..aea6b1900223f8ae7b33722176d99229ead77ec3 100644
--- a/src/gpu/GrGpu.h
+++ b/src/gpu/GrGpu.h
@@ -365,6 +365,13 @@ public:
                      const SkIRect& srcRect,
                      const SkIPoint& dstPoint);
 
+    const GrCaps::MultisampleSpecs& getMultisampleSpecs(GrRenderTarget* rt,
+                                                        const GrStencilSettings& stencil) {
+        using namespace std::placeholders;
+        const auto& functor = std::bind(&GrGpu::onGetMultisampleSpecs, this, rt, stencil, _1, _2);
+        return this->caps()->getMultisampleSpecs(rt->desc(), functor);
+    }
+
     struct DrawArgs {
         DrawArgs(const GrPrimitiveProcessor* primProc,
                  const GrPipeline* pipeline,
@@ -601,6 +608,12 @@ private:
                                const SkIRect& srcRect,
                                const SkIPoint& dstPoint) = 0;
 
+    // overridden by backend specific derived class to perform the multisample queries
+    virtual void onGetMultisampleSpecs(GrRenderTarget*,
+                                       const GrStencilSettings&,
+                                       int* effectiveSampleCnt,
+                                       SkAutoTDeleteArray<SkPoint>* samplePattern) = 0;
+
     void resetContext() {
         this->onResetContext(fResetBits);
         fResetBits = 0;