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Use almost the same color as video rush.

This is ugly and will have to go when InputSlots will be fixed (They always store the last displayed frame)

As soon as a clip is added to the timeline, render is started in paused mode.
This allows the user to move the position slider and gets an updated frame immediatly.

This reverts commit 86f08902.
This breaks render to a file. Please test your changes.

Fixes #129

Black output where returned because ClipWorkflow where too slow for the renderer.

Fixes #131