Optimize rect-shaped layers.

This brings back an optimization disabled in HC-MR1. This time the
correct geometry is generated to avoid unnecessary blending.

Change-Id: Id56404dc46bb84c75facc25c18488a690741b592
diff --git a/libs/hwui/Layer.h b/libs/hwui/Layer.h
index bb28437..26e240f 100644
--- a/libs/hwui/Layer.h
+++ b/libs/hwui/Layer.h
@@ -103,6 +103,11 @@
      * have been drawn.
      */
     Region region;
+    /**
+     * If the region is a rectangle, coordinates of the
+     * region are stored here.
+     */
+    Rect regionRect;
 
     /**
      * Color filter used to draw this layer. Optional.