Implement difference clip rects with window rectangles

src/gpu/GrClipStackClip.cpp

Index: src/gpu/GrClipStackClip.cpp
diff --git a/src/gpu/GrClipStackClip.cpp b/src/gpu/GrClipStackClip.cpp
index 0d6270f8d05e54036b374cc32b19978beac6f827..975f56a7579741fcbdb8001ff17841ade82f7ce1 100644
--- a/src/gpu/GrClipStackClip.cpp
+++ b/src/gpu/GrClipStackClip.cpp
@@ -7,9 +7,11 @@
 
 #include "GrClipStackClip.h"
 
+#include "GrAppliedClip.h"
 #include "GrDrawingManager.h"
 #include "GrDrawContextPriv.h"
 #include "GrGpuResourcePriv.h"
+#include "GrRenderTargetPriv.h"
 #include "GrStencilAttachment.h"
 #include "GrSWMaskHelper.h"
 #include "effects/GrConvexPolyEffect.h"
@@ -280,6 +282,23 @@ bool GrClipStackClip::apply(GrContext* context, GrDrawContext* drawContext, bool
 
     SkASSERT(reducedClip.hasIBounds());
 
+    // Attempt to implement difference clip rects with window rectangles. This will eventually
+    // become more comprehensive.
+    if (drawContext->accessRenderTarget()->renderTargetPriv().supportsWindowRectangles() &&
+        1 == reducedClip.elements().count() && !reducedClip.requiresAA() &&
+        InitialState::kAllIn == reducedClip.initialState()) {
+        const Element* element = reducedClip.elements().head();
+        SkRegion::Op op = element->getOp();
+        if (Element::kRect_Type == element->getType() &&
+            (SkRegion::kDifference_Op == op || SkRegion::kXOR_Op == op)) {
+            SkIRect window;
+            element->getRect().round(&window);
+            window.offset(-fOrigin);
+            out->addWindowRectangle(window);
+            return true;
+        }
+    }
+
     // An element count of 4 was chosen because of the common pattern in Blink of:
     //   isect RR
     //   diff  RR