This is a code refactor in preparation of supporting canvas
compositing and fix all cropping issues on full frame implementation.
No functional changes, all canvases are at (0, 0) position matching
tiled implementation.
File output node always received the resolution from the first socket.
When that socket didn't had a link it would use a resolution of 0,0.
What lead to not saving the file at all.
This only effected Multi layer OpenEXR files.
This change would go over all the links to find the first valid
resolution.