Add a control which allows overriding the gstreamer video sink.

This enables applications to selectively use platform specific or
unusual (video4linux perhaps) gstreamer sink elements to render video.

Change-Id: Ife2c2f9b2149d6b598c69a78f1e116ba05b46e10
Merge pull request #1 from adenexter/renderer-control

Add a control which allows overriding the gstreamer video sink.