videomixer: Fix handling of buffers without a duration
videomixer: Fix handling of buffers without a duration

We'll have to pop buffer from collectpads and store it
internally only to get the timestamp of the next buffer.
If we continue to keep it in collectpads, no new buffer
to calculate the end time will ever arrive.

https://bugzilla.gnome.org/show_bug.cgi?id=703743

Distrotech/gst-plugins-good,Distrotech/gst-plugins-good,Distrotech/gst-plugins-good,Distrotech/gst-plugins-good,Distrotech/gst-plugins-good