playout: New example for seamless audio/video playback

playout: New example for seamless audio/video playback

An example app that takes video URIs as command line arguments and switches
between them seamlessly one after the other using compositor and audiomixer.
Both audio-video and video-only media files are valid inputs, but mixing files
of both types in a single invocation is cumbersome to support, and hence does
not work. The example attempts to keep the audio stream moving along perfectly,
and duplicates video frames where necessary to cover gaps in the video
timestamps using the 'ignore-eos' videoaggregator pad property.

Ensuring seamless (and mostly-glitch-free) switching is harder than it sounds,
and hence the example contains plenty of pad probes and running time
calculations to make things work.

The GPtrArray play_queue contains items that are being played back, have been
prepared for playback, and will be played back in the future. The queue itself
is mutable besides the first two items (playing and prepared). The item that has
been prepared should not be edited or removed since it has been prepared in
advance to be activated immediately on the current item's EOS.

The example also has support for switching to the next item in the queue
prematurely; see the --switch-after/-s flag to the application.

Note: the output video is hard-coded at 1280x720, and input video is scaled as
needed to fit this size. Set OUTPUT_VIDEO_WIDTH/HEIGHT to change this.

https://bugzilla.gnome.org/show_bug.cgi?id=748947