Ignore negative screen physical size

Weston can send in some cases a negative physical size in the wl_output.geometry
event. While it is not clear whether that is legal go the extra mile and catch
it.

Change-Id: Ie2fbab84e653ad77732a72b6ca61509eb7849895
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>