[libinput] Improve touchscreen->output mapping heuristic
Our heuristic relies on knowing the physical sizes of the touchscreen and monitor.
However, not all monitors report a physical size, which makes it hard to map the touchscreen correctly.
Given a setup where we have one internal screen with known size, and an external display/touchscreen combination with unknown size we currently wrongly map the external touchscreen to the internal display.
However, in this case we know that this mapping is wrong since the size of the internal monitor and the touchscreen doesn't match. By then assigning the touchscreen to the other remaining display we can get the correct result