desktop: work around client-side rounding issues at right/bottom pixel

desktop: work around client-side rounding issues at right/bottom pixel

This also avoids a similar server-side rounding issue with some
combinations of wlroots and libwayland versions. See:

- https://gitlab.freedesktop.org/wayland/wayland/-/issues/555
- https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/5159

Tested with qmpanel (similar to lxqt-panel) at 1x and 2x output scale.
Does not help if scaling is done client-side, e.g. QT_SCALE_FACTOR=2.

Fixes: #2379
Fixes: #3099