Paint a rectangle on top of thumbnails indicating the run-time
of the video.
Use the z=100.0-101.0 range for painting the thumbnails, whereby
the z-value increases uniformly from first to last thumbnail
(sorted by timestamp). The duration-bars are placed at z-values
midway between those of the thumbnails.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
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