This was an attempt to avoid scratch gc creation and validation for paintwin
because that was expensive. This is not the case in current servers, and the
danger of failure to implement it correctly (as seen in all previous
implementations) is high enough to justify removing it. No performance
difference detected with x11perf -create -move -resize -circulate on Xvfb.
Leave the screen hooks for PaintWindow* in for now to avoid ABI change.
This new function walks the composite region and calls a rectangle
compositing function on each compositing rectangle. Previously there
were buggy duplicates of this code in fbcompose.c and
miext/rootles/safealpha/safeAlphaPicture.c.
