Remove the PaintWindow optimization.
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.
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