dashenc: Write segment timelines properly if the timeline has gaps

dashenc: Write segment timelines properly if the timeline has gaps

Write a new start time if the duration of the previous segment
didn't match the start of the next one. Check that segments
actually are continuous before writing a repeat count.

This makes sure timestamps deduced from the timeline actually
match the real start timestamp as written in filenames (if
using a template containing $Time$).

Signed-off-by: Martin Storsjö <martin@martin.st>