[Cogl] cogl-gst fixes/improvements and hardware decode stuff

[Lionel Landwerlin]

Hi there,

I recently started to play around again with hardware decode in
Clutter-Gst master and started to see problems and missing
functionalities in Cogl-Gst. The following patch set includes fixes
for some stuff already there, as well as new features to support
hardware decode.

This isn't the only thing we need to get hardware decode working out
of the box. People who contributed this patch set have identified bugs
in various places like Gst-vaapi or Gstreamer plugins, but it's a good
step forward to get stuff to work.

Cheers,

-
Lionel