videorate: Do not loop forever pushing first buffer when variable framerate

videorate: Do not loop forever pushing first buffer when variable framerate

In the case the framerate is variable (represented by framerate=0/1),
we currently end up loop pushing the first buffer and then recompute
diff1 and diff2 without updating the videorate->next_ts at all
leading to infinitely looping pushing that first buffer.

In the case of variable framerate, we should just compute the next_ts
as previous_pts + previous_duration.

https://bugzilla.gnome.org/show_bug.cgi?id=734424