Properly reject empty unclipped savelayers

bug:27225580
bug:27281241

Empty unclipped savelayers (clipped at defer time, often by dirty rect)
were resulting in invalid layer clear rectangles. Simplify by just
rejecting these unclipped savelayers entirely at defer.

Also, use repaint rect as base clip for constructed ops within
LayerBuilder.

Change-Id: I5c466199e85201a2f68f5cdc60b29187c849961b