Gstreamer: Use GstVideoOverlay if possible

Currently when gstreamer is used to decode a full-screen
stream sent from the server, the decoding frames are being
forced to RBGA format and pushed using appsink to be scaled
and rendered to screen.

Today most of the gstreamer sinks supports the GstVideoOverlay
interface which allows to render directly from the pipeline to
a window by a given windowing system id (i.e. xid). This patch
makes playbin to use this feature if possible.

Setting the DISABLE_GSTVIDEOOVERLAY environment variable will
make gstreamer to avoid of using the gstvideooverlay interface.

Acked-by: Frediano Ziglio <fziglio@redhat.com>