Overdraw avoidance in new pipeline

bug:27873093

Adds the simple overdraw avoidance optimization to the new
pipeline. This means when LayerBuilder defers draws that are opaque over
the full area of the repaint region, it will discard all drawing content
beneth.

Also moves a lot of complexity out of BakedOpState's header.

Change-Id: Iffca6d8e1b170ef31a5d6c83d25592670e02323d