cogl-gst: video-sink: rework caps exposure
cogl-gst: video-sink: introduce specific min/mag filters per layers
cogl-gst: video-sink: add NV12 support

Based on a patch from Edward Hervey and Matthieu Bouron for
Clutter-Gst :

https://bugzilla.gnome.org/show_bug.cgi?id=712832
cogl-gst: video-sink: fix not installed rgb shader
cogl-gst: video-sink: use intended GStreamer API for data/stride/etc...

Based on a patch for ClutterGst from Matthieu Bouron <matthieu.bouron@collabora.com>
cogl-gst: video-sink: add shutdown method on renderers
cogl-gst: video-sink: add gl upload support
cogl-gst: video-sink: use caps feature define rather that raw string
cogl: winsys: glx: fix valgrind complaint
cogl-gst: video-sink: add special filter for hardware decoded frames

Hardware decoded frames are unlikely to be mipmapped. We can't use the
same filters as the ones for CPU decoded/uploaded frames.
cogl-gst: video-sink: add video balance support