In cases where the value is actually valid, the input device lives
just as long as the output, so it's easier to provide the output itself
instead for making a compositor find it by the name.
Currently, the "wlr_event_pointer_axis" event stores low-resolution
values in its "delta_discrete" field. Low-resolution values are always
multiples of one, i.e., 1 for one wheel detent, 2 for two wheel
detents, etc.
In order to simplify internal handling of events, always transform in
the backend from the low-resolution value into the high-resolution
value.
The transformation is performed by multiplying by 120. The 120 magic
number is used by the kernel and it is exposed to clients in the
"WLR_POINTER_AXIS_DISCRETE_STEP" constant.
All the code logic related to the pointer has been moved to its own file.
The seat is responsible for the lifetime of its wlr_wl_pointer(s), and assigning
them to the relevant wlr_wl_output. The wlr_wl_pointer becomes a simple helper
to manager the wlr_pointer associated to the seat's wl_pointer and its lifetime.
