This is for debugging purposes only, we cannot guarantee that event timestamps
always go up - at least not across devices. Example: tapping on a touchpad may
delay an event until a timeout expires, but that event is then sent with the
original touch timestamps (i.e. in the past). If any other device produces
events during that timeout period, our timestamps are out-of-order.

This isn't really a bug because we are forced to do that, but for bug-fixing
it can be useful to detect.

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