Support mapping input devices to outputs

[ifreund]

If using a laptop with a touchscreen and an external monitor, it is desirable to have the touchscreen inputs interpreted relative to the laptop screen specifically. I'm not aware of any way to do this mapping automatically, so we probably need something akin to sway's map_to_output input configuration.

[ghost]

Would something along the lines of buffet/kiwmi#65 work for automatic mapping?

[ifreund]

@tiosgz Hmm, yes something like that should be implemented for pointer and touch input devices to make the X11/Wayland backends work properly. That wouldn't solve this issue however, which is about the DRM backend where the input device's output_name is not set by wlroots afaik.