avdemux: update the context we use to determine stream's caps

The discovered frame rate is only available on the AVStream
itself. Updating the temporary context framerate before
building caps from it seems like a pretty non-intrusive approach.

Fixes #75

Part-of: <gstreamer/gst-libav!74>
23 jobs for !74 with avdemux-fix-fps in 26 minutes and 28 seconds (queued for 2 seconds)
merge request