dashenc: Fix writing of timelines that don't start at t=0

When writing an explicit time, reset the cur_time variable to this
value as well. This avoids writing excessive time attributes for each
segment in the timeline, as long as the segments are continuous.

Signed-off-by: Martin Storsjö <martin@martin.st>
dashenc: Remove some stray double spaces

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