Comparison: markers should be used to synchronize only

At the moment the markers (or one of them?) also define the beginning of the data comparison when you start a new comparison job. But they should just be used to synchronize the compared videos (to define the offset). This will allow users to look for a significant movement within the movement sequence instead of finding the starting point that might be hard to recognise (especially visually in the video). The comparison should then be applied to the whole video (all overlapping time).