This is to handle cases where upstream handles the fragmented streaming in TIME
segments and sends us data with gaps within fragments. This would happen when dealing
with trick-modes.

When upstream (push-based, TIME SEGMENT) wishes to send discontinuous samples,
it must obey the following rules:
* The buffer containing the [moof] must have a valid GST_BUFFER_OFFSET
* The buffers containing the first sample after a gap:
 * MUST start at the beginning of a sample,
 * MUST have the DISCONT flag set,
 * MUST have a valid GST_BUFFER_OFFSET relative to the beginning of the fragment.

https://bugzilla.gnome.org/show_bug.cgi?id=767354