Enable previews for inactive compo output and viewer nodes.

This keeps old behavior and could be nice. However, it'll
make compositing slower  (doing final compo after editing
tree). This is easy to hide preview in this case, and also
i hope to implement percentage rendering soon.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55501 954f8c5b-7b00-dc11-b283-0030488c597c