Aiming at fixing a rare race condition where we get:
../subprojects/gstreamer/libs/gst/check/gstcheck.c:1258:F:nlecomposition:test_seek_on_nested:0: nested_src0_0x1a1a310 is not destroyed, 1 refcounts left!
The idea is that there might have a remaining GstMessage
with the nested_src as `message.src` on the bus that has
yet to be processed in some conditions leading to a reference
still existing when unrefing the pipeline.
Otherwise if we shutdown a composition whith an nested composition
(inside a source in the test) and leak it, we end up with the nested
composition task still running (in READY) which is bad.
Add a test for that which leaks the pipeline on purpose.
GstDiscoverer objects were leaked by tests making the leaks detector
unusable.
Introduce ges_deinit(), similiar to gst_deinit(), doing some cleanup
before exiting the process.
https://bugzilla.gnome.org/show_bug.cgi?id=776805
The strategy here is to seek at the new end of the composition. And in
GES we always add a 1ns long gap at the end of the tracks so that all
track have the exact same duration, and we have black frames when the
timeline is empty
