Fix problems resulting in left-over queued frames

* Use -1 rather than 0 as a flag for pending queue entries; 0 is
  a valid frame_counter value from Cogl.
* Consistently handle the fact we can have more than one pending
  entry. It's app misbehavior to submit a new frame before
  _NET_WM_FRAME_DRAWN is received; but we accept such frame messages,
  so we can't just leak them.
* If we remove send_frame_message_timer, assign the current frame counter
  to pending entries.
* To try to avoid regressing on this, when sending _NET_WM_FRAME_TIMINGS
  messages, if we have stale messages, or messages with no frame drawn
  time, warn and remove them from the queue rather than just accumulating.
* Improve commenting.

https://bugzilla.gnome.org/show_bug.cgi?id=738686
MetaWindowActor: don't overwrite send_frame_messages_timer

If the app finished multiple frames before we sent _NET_WM_FRAME_DRAWN,
we could add the send_frame_messages_timer multiple times. In the rare
case that the app immediately closed the window, the older timeout
could potentially then run on the freed actor.

https://bugzilla.gnome.org/show_bug.cgi?id=738686