Map touch inputs to the single enabled display when the 'last' multi-display mode is in use

[matt-oakes]

I'm not certain if this is something you will want to merge as-is, but it was a useful patch for us to resolve issues with touch input devices which do not report which display out they map to.

In our situation we run with -m last, so there is only ever a single enabled display. This PR detects this situation and automatically maps the touch and pointer inputs to this enabled display.

This avoids the need for pre-configuring this using udev rules (I beleive this is considered deprecated anyway?) and allows Cage to automatically work with any touch input device and output display automatically.

I have made sure this works both when the input devices are signalled as added before the outputs and when either the inputs or outputs change.

Related to #243.

Similar in idea to #167 and #193.

[joggee-fr]

I thought that without mapping reported by the (touch) input, it was mapped to the whole output layout. Consequently, if you only have one display enabled, it should be always right.
That was not the case for you @matt-oakes?

[matt-oakes]

That is true, which means it works fine if the output has not transforms applied to it. However, as it's not actually linked to the output itself, it doesn't take into account any transforms which are active on the display. This means that if you have a rotated or flipped display, the touch will be applied in the wrong location.

In our situation we allow have touch displays which can change orientation and rotation so we need to account for that.

This PR makes the link between the input device and the output device properly so transforms are taken into account.

[joggee-fr]

@matt-oakes
Ok, I got it. Thanks for the precision.

This comment from @emersion gives some ideas for input to output mapping. You PR is a very special case of the heuristic solution I guess.

[matt-oakes]

I've just looked again at the wlroots code, and there might actually be a better and more "correct" way to achive the same outcome as this PR does.

When an absolute touch event comes in, it gets the mapped output and transforms it.

When getting the mapped output, it first checks the mapped_output of the input device (the property which this PR currently sets). If that isn't set, it then checks the state->mapped_output property of the wlr_cursor itself.

We can set this by calling wlr_cursor_map_to_output. We would do this whenever we change the single enabled display. This would happen at the same points we're currently calling seat_remap_inputs_to_first_enabled_output in this PR.

I think this actually is the correct behaviour for -m last mode anyway as we would be saying "the cursor is mapped to this single output". Because of this, we wouldn't need to make any changes to the current calls to wlr_cursor_map_input_to_output so if an input device does correctly set the wlr_output propert, this would still be set correctly. It would only fall through to the cursor->state->mapped_output when an input device doesn't have a mapped_output set.

Does that sound like the correct behaviour to you too?