videomixer: Fix synchronization if dynamically changing the FPS
videomixer: Fix synchronization if dynamically changing the FPS

https://bugzilla.gnome.org/show_bug.cgi?id=735859

wkatsak/gst-plugins-good,vatavuserban/gst-plugins-good,jpakkane/gstreamer-plugins-good,surround-io/gst-plugins-good,ndufresne/gst-plugins-good,wkatsak/gst-plugins-good,ndufresne/gst-plugins-good,wkatsak/gst-plugins-good,surround-io/gst-plugins-good,vatavuserban/gst-plugins-good,jpakkane/gstreamer-plugins-good,surround-io/gst-plugins-good,ndufresne/gst-plugins-good,jpakkane/gstreamer-plugins-good,wkatsak/gst-plugins-good,vatavuserban/gst-plugins-good,vatavuserban/gst-plugins-good,surround-io/gst-plugins-good,ndufresne/gst-plugins-good,jpakkane/gstreamer-plugins-good