[Cogl] gst:Implement frame grab behaviour

[Neil Roberts]

Thanks Plamena! I've pushed this to the wip/cogl-gst branch along with
some extra patches that I'm also posting here.

I noticed that the code for the hardware renderer looks broken. For
example it is still directly setting the texture on the pipeline and
it doesn't make a copy. Also it initialises a GValue like this:

 g_value_init (&value, gl_texture);

The second argument g_value_init() is supposed to be a GType so
presumably this won't work.

The hardware renderer is guarded behind #ifdef HAVE_HW_DECODER_SUPPORT
but as far as I can tell this define isn't set anywhere so presumably
it's not being used. It'd be good to make this work properly at some
point seeing as it's a feature in ClutterGst.

Regards,
- Neil