Take a snapshot of the time every 10 libinput_dispatch() calls. During event
processing, check if the event timestamp is more than 10ms in the past and
warn if it is. This should provide a warning to users when the compositor is
too slow to processes events but events aren't coming in fast enough to
trigger SYN_DROPPED.

Because we check the device event time against the dispatch time we may get
warnings for multiple devices on delayed processing. This is intended, it's
good to know which devices were affected.

In the test suite we need to ignore the warning though, since we compose the
events in very specific ways it's common to exceed that threshold
(particularly when calling litest_touch_move_to).

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>