backends: Check both input settings and mapper for tablet monitors
The upper layers (OSDs basically) want to know the monitor that a tablet is currently assigned to, not the monitor just as configured through settings.
This broke proper OSD positioning for display-attached tablets since commit 87858a4e, as the MetaInputMapper kicks in precisely when there is no configured monitor for the given device.
Consulting both about the assigned output will make OSDs pop up again in the right place.