decode: fix creation of GLX video buffers for GStreamer 0.10.
decode: fix creation of GLX video buffers for GStreamer 0.10.

Fix creation of GstVaapiVideoBuffer objects (i) to have that type for real;
and (ii) to correctly extract the GstSurfaceConverter from the video buffer
object meta.

This fixes support for cluttersink with GStreamer 0.10 builds.

01org/iotg-lin-gfx-gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,CapOM/gstreamer-vaapi,CapOM/gstreamer-vaapi,GStreamer/gstreamer-vaapi,CapOM/gstreamer-vaapi,ceyusa/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi