Blender Git Loki

Sequencer: disabled sequencer recursion and corrected rendered preview mode

Sequencer recursion was never actually supported and only gives lots of
issues. Disabled it now, so users are not getting confused by semi-working
stuff.

Also made a correction to rendered sequencer preview, so now using scene
strip in it's own sequencer will work properly (it produced black frames
before).

This required killing compositor jobs since they could be using the same
render result as renderer called from sequencer uses.

Small improvements could be:
- Add slight delay before compositor job starts handling nodes so killing
this job would be fast
- Tag compositor to be updated after preview was fully rendered.