Implement difference clip rects with window rectangles

src/gpu/gl/GrGLIRect.h

Index: src/gpu/gl/GrGLIRect.h
diff --git a/src/gpu/gl/GrGLIRect.h b/src/gpu/gl/GrGLIRect.h
index 41ac13b753eecfc996d9110f4c90969593836989..a699ae36bd79bc54cd793d1d46c114ed9cebe59a 100644
--- a/src/gpu/gl/GrGLIRect.h
+++ b/src/gpu/gl/GrGLIRect.h
@@ -23,6 +23,20 @@ struct GrGLIRect {
     GrGLsizei fWidth;
     GrGLsizei fHeight;
 
+    /**
+     *  cast-safe way to treat the rect as an array of (4) ints.
+     */
+    const int* asInts() const {
+        return &fLeft;
+
+        GR_STATIC_ASSERT(0 == offsetof(GrGLIRect, fLeft));
+        GR_STATIC_ASSERT(4 == offsetof(GrGLIRect, fBottom));
+        GR_STATIC_ASSERT(8 == offsetof(GrGLIRect, fWidth));
+        GR_STATIC_ASSERT(12 == offsetof(GrGLIRect, fHeight));
+        GR_STATIC_ASSERT(16 == sizeof(GrGLIRect)); // For an array of GrGLIRect.
+    }
+    int* asInts() { return &fLeft; }
+
     void pushToGLViewport(const GrGLInterface* gl) const {
         GR_GL_CALL(gl, Viewport(fLeft, fBottom, fWidth, fHeight));
     }
@@ -39,6 +53,11 @@ struct GrGLIRect {
     // sometimes we have a SkIRect from the client that we
     // want to simultaneously make relative to GL's viewport
     // and (optionally) convert from top-down to bottom-up.
+    void setRelativeTo(const GrGLIRect& glViewport, const SkIRect& devRect, GrSurfaceOrigin org) {
+        this->setRelativeTo(glViewport, devRect.x(), devRect.y(), devRect.width(), devRect.height(),
+                            org);
+    }
+
     void setRelativeTo(const GrGLIRect& glRect,
                        int leftOffset,
                        int topOffset,