Frame preview+speed bug

[amindfv at gmail.com]

I'm not running bleeding-edge kdenlive, so I can't be sure this hasn't been fixed. But with v18.12.3 this is definitely an issue. Repro:

  - Create a new project
  - Add a long video to the timeline
  - Use the razor tool to cut the video somewhere, e.g. halfway through
  - Add a Speed effect to the second (later) clip - set it to 200%
  - Zoom all the way in on the timeline

The frame previews are all from a previous point in the video. E.g. with a cut 20 mins in and at 200% speed, the frame previews are from the 10 minute point in the video.