Use the thumb producer form project clip in ThumbJob

Summary:
Update the ProjectClip::thumbProducer to be a std::shared_ptr and use it in the
ThumbJob.

Using the originalProducer() causes crashes when using movit acceleration.

Test Plan:
- Enable movit
- Run kdenlive
- Add a JPEG image to the project bin

Reviewers: alcinos, mardelle

Reviewed By: alcinos, mardelle

Differential Revision: https://phabricator.kde.org/D8808