Edward Hervey authored Aug 12, 2016 and Edward Hervey committed Aug 12, 2016

In ringbuffer mode we need to make sure we post buffering messages *before*
blocking to wait for data to be drained.

Without this, we would end up in situations like this:
* pipeline is pre-rolling
* Downstream demuxer/decoder has pushed data to all sinks, and demuxer thread
  is blocking downstream (i.e. not pulling from upstream/queue2).
* Therefore pipeline has pre-rolled ...
* ... but queue2 hasn't filled up yet, therefore the application waits for
  the buffering 100% messages before setting the pipeline to PLAYING
* But queue2 can't post that message, since the 100% message will be posted
  *after* there is room available for that last buffer.

https://bugzilla.gnome.org/show_bug.cgi?id=769802