This adds several utility apis that aim to make it as easy as possible
for an application to determine what size a video should be drawn at.
The important detail here is that these apis take into account the
pixel-aspect-ratio in addition to the video's own aspect ratio.
This patch updates the cogl-basic-video-player example to use the
cogl_gst_video_sink_fit_size() api to perform letterboxing.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
(cherry picked from commit d26f17c97ff6b9f6d6211e0527d5965a85305a56)