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.
The former <X11/extensions/XKBsrv.h> has been pulled into the server now as
include/xkbsrv.h, and the world updated to look for it in the new place,
since it made no sense to define server API in an extension header. Any
further work along this line will need to do similar things with XKBgeom.h
and friends.
