Fix #35439: Render Border have weird influence on compositing

Issue was caused by render result passing to a compositor
was having a resolution of border when using a camera border
in cases there's no Render Layer nodes.

Made it so resolution in which compositor works does not
depend on whether Render Layers present or not.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57604 954f8c5b-7b00-dc11-b283-0030488c597c