Drain all events before synchronizing after SYN_DROPPED

Drain all events before synchronizing after SYN_DROPPED

The kernel ring buffer drops all events on SYN_DROPPED, but then continues to
fill up again. So by the time we read the events, the kernel's client buffer is
essentially like this:
  SYN_DROPPED, ev1, ev2, ev3, ...., evN

The kernel's device state represents the device after evN, and that is what
the ioctls return. For EV_KEY, EV_SND, EV_LED and EV_SW the kernel removes
potential duplicates from the client buffer [1], it doesn't do so for EV_ABS.

So we can't actually sync while there are events on the wire because the
events represent an earlier state. So simply discard all events in the kernel
buffer, synchronize, and then start processing again. We lose some granularity
but at least the events are correct.

[1] http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/drivers/input/evdev.c?id=483180281f0ac60d1138710eb21f4b9961901294

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