This field's ownership is unclear: it's in wlr_input_device, but
it's not managed by the common code, it's up to each individual
backend to use it and clean it up.
Since this is a backend implementation detail, move it to the
backend-specific structs.
This commit makes `get_current_time_msec` correctly return milliseconds
as opposed to microseconds. It also considers the value of `tv_sec`, so
we don't lose occasionally go back in time by one second. Finally, the
function is moved into `util/time.cc` so that it can be reused elsewhere
without having to consider these pitfalls.
When calling assert(0) instead of returning a value, -Wreturn-type
warnings are triggered because assertions can be disabled. Replace these
assertions with abort().
This allows wlroots based compositors to properly use graphic tablets
with the wayland backend.
This should be a decent quality of life improvement when working on
tablet related features.
