The previous values happened to work in basic cases, but not in general
if the destination is a subwindow or has a border.
Fixes crash with xli, which moves a large subwindow inside a smaller
parent window for scrolling.
No regressions with xterm, x11perf -copyplane or the xscreensaver
phosphor hack.
Bug: https://bugs.debian.org/857983
Reviewed-by: Keith Packard <keithp@keithp.com>