Fix for recent compo border commit

Viewer operations shall be aware of border as well, otherwise
CPU would be busy for a while full compo isn't done for just
a small preview image in a node.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55135 954f8c5b-7b00-dc11-b283-0030488c597c