qtdemux: Add support for the mp2v fourcc for MPEG-2 video

https://bugzilla.gnome.org/show_bug.cgi?id=709270
v4l2src: print probed caps as caps again in debug log

This got lost during refactoring.
videomixer2: save tag events to send them in collected.

Otherwise there were race conditions where we would send tags
on a flushing srcpad.

Based on gstadder's logic.
videomixer: Check if the pad needs reconfiguration in collected.
videomixer: update videoconvert copy.