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This also solves a "lag" problem.

However, at the moment, the first frame isn't displayed... we need to move the position slider to get one...

IE you can pause the preview, but still change the position, and the render gets updated.

This is really much cleaner than the old versions...

An empty zone in the timeline returns a black screen now (it was returning the last renderer picture until now...)

However, due tu the nature of clipworkflow control, a lot of frames are
lost...

Performances when moving along in the clip should be a little improved
in synchrone mode.

This works for adding/removing clip