When using border rendering, use the same border for compositor

This makes compositing as fast as it's possible in this case.

The only thing is border render+crop will still give funcy
results. This is the next thing to be solved in compositor.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55450 954f8c5b-7b00-dc11-b283-0030488c597c
Changes to compositor output node

Make it so compositor output node wouldn't be calculated
when Render Result image is not visible on the screen.
This makes compositor tree editing more friendly and
faster.

Also, if there's no viewer image visible on the screen
viewer nodes wouldn't be handled.

Final rendering keeps unchanged for now.

This solves issues when for performance artists are
disconnecting compo output node before tweaking values
in compositor and forgets to attach compo output
node before sending file to the farm.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55451 954f8c5b-7b00-dc11-b283-0030488c597c