Initialize MergingDrawBatch clip with viewport bounds

This allows merged, clipped operations to behave correctly within a
savelayer, even if the base viewport has a large offset.

Additionally, disregard opaqueness when within a
complexclip/savelayer, as the coverage can't be trusted.

Change-Id: Ic908b82a4bb410bc7fac1b4295f4874ed166efc5
diff --git a/libs/hwui/OpenGLRenderer.h b/libs/hwui/OpenGLRenderer.h
index 548501d..aa0f9fc 100644
--- a/libs/hwui/OpenGLRenderer.h
+++ b/libs/hwui/OpenGLRenderer.h
@@ -360,6 +360,9 @@
         return mSnapshot->clipRegion->isEmpty();
     }
 
+    int getViewportWidth() { return getSnapshot()->viewport.getWidth(); }
+    int getViewportHeight() { return getSnapshot()->viewport.getHeight(); }
+
     /**
      * Scales the alpha on the current snapshot. This alpha value will be modulated
      * with other alpha values when drawing primitives.