Allow custom GStreamer elements to be used as a video sinks.

Change-Id: I91b489adb92d69d7edc8e8c745fb0d1c23a2e891
Allow the camera source to be overridden with an environment variable.

And only intervene in caps negotiation when specific values have been
set, otherwise we're just as likely to create a caps filter which
prevents the pipeline being setup as resolve indecision between src
and sink elements.
Fix playback with exotic decoders.

Allow overriding of flags, and extend VAAPI hack to additional formats.
Merge pull request #2 from adenexter/gstreamercontrol

[qtmultimedia] Enable the use of non-standard gstreamer cameras, and video sinks.