Fix #35144: Image editor is not updateing if in new window

Was a regression in own optimization on which viewer node to
update -- need to check all the windows' screens, not just
active one.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56404 954f8c5b-7b00-dc11-b283-0030488c597c