[gnlcomposition] Creating too many segments when not needed.

Submitted by Edward Hervey `@bilboed`

+++ This bug was initially created as a clone of Bug 609792 +++

The problem is that this composition will work:

[ overlaid videomixer gnloperation]
[ source 1 ]
[ source 2 ]

Creating the 3 proper segments (oper+source1, oper+source1+source2, oper+source2)

But it also creates 3 segments for the following:
[ source 1 ]
[ source 2 ]

While it should only create 2 (the whole of source1, followed by the rest of source2).

This is a reminder so we don't forget to fix this for performance reason (to avoid useless seeking/segments).