Stop DisplayPollThread in StopProcessingEvents.

StartProcessingEvents starts the thread with options and starts polling.
StopProcessingEvents stops polling for wayland events but doesn't
actually stop the thread. This patch fixes it.
This is based on initial work done here:
https://github.com/baillaw/ozone-wayland/commit/142abd2525a39ef2ad5c356e9a53987d35039a98