Theora and vorbis use running time (which is correct) for calculating
the granulepos for their ogg packets. Oggmux, however, used
timestamps to order the received buffers.

This patch makes it use the running time to compare buffer times
and also to timestamp pushed buffers.

Some bits of the code still use timestamps, but they are only
used to calculate durations, so it should be fine.

https://bugzilla.gnome.org/show_bug.cgi?id=643775