Fix #34205: Cycles: Zooming in rendered mode during update out of sync with intended zoom

Made sure no pause_cond.wait() happens at the same time as session as resetting.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54491 954f8c5b-7b00-dc11-b283-0030488c597c