video-sink: Add NV12 shader support

Based on patch from Lionel Landwerlin.

https://bugzilla.gnome.org/show_bug.cgi?id=712832
video-sink: support video meta api

Support of the video meta API (including buffers alignment) is done
through the video frame API.

https://bugzilla.gnome.org/show_bug.cgi?id=712834
video-sink: add context field to GstClutterSinkRenderer

https://bugzilla.gnome.org/show_bug.cgi?id=712834
video-sink: suport gl texture upload meta api

https://bugzilla.gnome.org/show_bug.cgi?id=712834
video-sink: support video crop meta api in GLTextureUploadRenderer

https://bugzilla.gnome.org/show_bug.cgi?id=712834