Bug 1520909: Properly support SupportsRegionClipping in DrawTargetDual. r=mattwoodrow

Differential Revision: https://phabricator.services.mozilla.com/D16899

--HG--
extra : rebase_source : 03ad075eb41e13b64686dd9939fe5dbcbb6e107e
extra : amend_source : 3ded96951d4ad113cbd55ff96c8cfbf8a4f4273a

gfx/2d/DrawTargetDual.h

@@ -69,6 +69,10 @@ class DrawTargetDual : public DrawTarget {
     mB->SetTransform(aTransform);
   }
 
+  virtual bool SupportsRegionClipping() const override {
+    return mA->SupportsRegionClipping() && mB->SupportsRegionClipping();
+  }
+
   virtual void DrawSurface(SourceSurface *aSurface, const Rect &aDest,
                            const Rect &aSource,
                            const DrawSurfaceOptions &aSurfOptions,