Michel Dänzer authored Jul 25, 2018 and Michel Dänzer committed Aug 16, 2018

Instead of processing DRM events directly from drmHandleEvent's
callbacks, there are three phases:

1. drmHandleEvent is called, and signalled events are re-queued to
   _signalled lists from its callbacks.
2. Signalled page flip completion events are processed.
3. Signalled vblank events are processed.

This should make sure that we never call drmHandleEvent from one of its
callbacks, which would usually result in blocking forever.