Blender Git Loki

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.