Fr. Steve
Back to your original question.
A compositor is a type setter; he or she used a composing stick, which has a capacity to hold one line of type. The good compositors never took their eyes off the copy, and their grabbing hands unerringly found the correct compartment and inserted the type without looking at it. The end of the type opposite the letter had a little hook or ell-shape on it so the compositor could set it into the stick without looking at it.
But how does that become compositing (and what IS compositing.)
This URL http://graphics.stanford.edu/courses/cs248-95/comp/comp.html leads you to a set of lecture notes on compositing. Much if it's beyond my ken, but there is a definition of digital compositing, and it's apparently a computer graphics term.
Compositing appears likely to me to be a back-formation from compositor.
