glamor: Fix temporary pixmap coordinate offsets

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>
(cherry picked from commit ffda82ed)