Blender Git Loki

Fix Cycles X shadow catcher artifacts

Were caused by matte object appearing in the division process.

This change makes it so the matte object is removed from the
combined pass before division, which solves artifacts caused
by anti-aliasing (the matte "bleeds" into the shadow catcher
and produces artifacts around the rim).

Additionally made certain parts more explicit and better
documented.

Test file: {F10177353}

| Before | After |
| {F10177355} | {F10177358} |

Differential Revision: https://developer.blender.org/D11632