Overdraw avoidance and merging of clipped ops

bug:8951267

If an opaque op, or group of opaque ops covers the invalidate region,
skip draw operations that precede it.

Clipped operations may now be merged, but only if they share a
clipRect - this is a very case for e.g. ListView, where all background
elements may now be a part of the same MergingDrawBatch.

It is this more aggressive merging that groups together clipped
background elements in the ListView case, enabling the overdraw
avoidance skipping the window background.

Change-Id: Ib0961977e272c5ac37f59e4c67d828467422d259