We didn't tear down the libinput objects in the right thread when
exiting, but did so after the input thread exited.
We also tried to destroy the libinput devices after the libinput context
was destroyed, which isn't allowed.
Fix these two issues by tearing down the libinput objects in a input
thread task that when done exits the input thread. This effectively
"flushes" the input thread tasks while destroying the libinput objects
just before the thread exits.
While it might fine to tear down libinput objects in an arbitrary (main
in this case) thread while making sure nothing pokes at it in parallel
(e.g. the input thread is gone), libinput is by definition single
threaded, and could theoretically make assumptions about this, and we
shouldn't cause any possible surprises here, so make sure to destroy it
all in the right thread.
This fixes an abort() on exit caused by an assert about invalid object
destruction in libinput.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1663>
237cdeef4c