omx: wait for flush complete and buffers being released when flushing

omx: wait for flush complete and buffers being released when flushing

When flusing we should wait for OMX to send the flush command complete event
AND all ports being released.
We were stopping as soon as one of those condition was met.

Fix a race between FillThisBufferDone/EmptyBufferDone and the flush
EventCmdComplete messages. The OMX implementation is supposed to release
its buffers before posting the EventCmdComplete event but the ordering
isn't guaranteed as the FillThisBufferDone/EmptyBufferDone and
EventHandler callbacks can be called from different threads (cf 2.7
'Thread Safety' in the spec).

Only wait for buffers currently used by OMX as some buffers may not be
in the pending queue because they are held downstream.

https://bugzilla.gnome.org/show_bug.cgi?id=789475