cogl-gst: video-sink: add NV12 support
cogl-gst: video-sink: add NV12 support

This adds a cogl-gst renderer to decode NV12 data. NV12 is split into
two buffers, one for the luma component and another for the two
chrominance components at a quarter of the resolution. The second
buffer is uploaded to a two-component RG texture. RG-component
textures are only supported if COGL_FEATURE_ID_TEXTURE_RG is
advertised by Cogl so the NV12 cap is also only advertised when that
is available.

Based on a patch by Lionel Landwerlin which was in turn based on a
patch from Edward Hervey and Matthieu Bouron for Clutter-Gst:

https://bugzilla.gnome.org/show_bug.cgi?id=712832

Reviewed-by: Robert Bragg <12e9293ec6b30c7fa8a0926af42807e929c1684f@linux.intel.com>
Reviewed-by: Lionel Landwerlin <c05e47e57584a5194aa13fa9b87ac49f6ecd5024@gmail.com>

djdeath/cogl,djdeath/cogl,djdeath/cogl