qtmux: Set Closed Caption track width/height to that of the first video track
qtmux: Set Closed Caption track width/height to that of the first video track

Otherwise software like Premiere or Final Cut Pro won't like our files.

https://bugzilla.gnome.org/show_bug.cgi?id=797111

StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good