Now that we have the full render nodes available, there is not much
benefit in fine-grained control over multiple rectangles.
In particular, it's causing pain with complex regions.
There might be a benefit in clipping to the region's rectangles in cases
like widget-factory where the whole diff is made up of the 2 rectangles
of spinner and the pulsing progress bar, but it needs a good heuristic
for where this is useful.
