Support shadows and reordering in ViewOverlay

        @Override

        protected void dispatchDraw(Canvas canvas) {

            /*

             * The OverlayViewGroup doesn't draw with a DisplayList, because

             * draw(Canvas, View, long) is never called on it. This is fine, since it doesn't need

             * RenderNode/DisplayList features, and can just draw into the owner's Canvas.

             *

             * This means that we need to insert reorder barriers manually though, so that children

             * of the OverlayViewGroup can cast shadows and Z reorder with each other.

             */

            canvas.insertReorderBarrier();

            super.dispatchDraw(canvas);

            canvas.insertInorderBarrier();

            final int numDrawables = (mDrawables == null) ? 0 : mDrawables.size();

            for (int i = 0; i < numDrawables; ++i) {

                mDrawables.get(i).draw(canvas);