Hi all,

this patch adds screen subtraction to the keying node. This makes it far more useful in my opinion,
because you immediately get a result that is premultiplied much more correctly than just sticking
a convert alpha node after the keying node. This could be easily replicated by having a subtract node
after the key, but integrating it into the keying node has the benefit of being able to use the clip black/clip white
ranges and feather/dilate options correctly. Integrating it also makes it much more easy to use
for new users, as screen subtraction is probably what you want to do anyways after pulling a key.

hope this helps, David.