Blender Git Loki

Fix alpha in denoised shadow catcher pass in Cycles X

Seems it was wrong since the initial implementation.

The issue is that for the shadow catcher pass we can not copy alpha
from input pass as the pass is calculated based on other passes.

Decided to go with an implicit knowledge that composited passes are
always opaque. Saves some complications and memory by storing full
RGBA buffer for noisy composited passes.

Differential Revision: https://developer.blender.org/D11825