Fix #33822: Compositor gets stuck if a sequencer window is also visible

Issue was caused by a fix for rendered sequencer preview mode, which
will likely conflict with compositor job. Made it so compositor job
will be killed when sequencer uses rendered preview.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53709 954f8c5b-7b00-dc11-b283-0030488c597c