mxfdemux: Handle temporal offset with PTS < DTS

This is similar to how the same issue was handled in qtdemux.

In order for the "DTS <= PTS" constraint to be respected, we calculate the maximum temporal reordering that can happen (via index tables).

If there is a non-0 temporal reordering, we:

• Shift all outgoing PTS by that amount
• Shift segment for that stream by that amount
• Don't modify DTS (i.e. they might end up having negative running-time, before the start of the segment)

The final PTS for an index should also include the temporal offset of the index. In a temporal reordering, buffers are now having corrrect PTS out of the parser/decoder

Fixes:

• #584 (closed)

• gst-plugins-base#895 (closed)

(and maybe a lot of other issues)

Forked from !2338 (closed)