Port gstreamer capture plugins, video renderer.

As a side effect of not porting the same code two or three times over
this moves a lot of duplicated and often out of sync code to QGstUtils.
It also adds a new renderer plugin interface as the buffer allocation apis
have changed and aren't compatible with QGstBufferPoolInterface.

Change-Id: I0a86a5449600df8574eff6f831ce1591d8dd29db
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>
Add missing #include.

GstVideoInfo is no longer available through an indirect include.

Change-Id: I57f22b31420da1b6eca0658b97e876a5105c9b01
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>