Blender Git Loki

Fix wrong update with shadow catcher and transparent film

This change fixes an issue when scene has a shadow catcher and film is
configured to be transparent. Starting viewport render and making the
background non-transparent will cause bad memory access (wrong render
and possibly crash).

Film passes depends on transparency of background, so check for this.

Demo file: F10650585

Differential Revision: https://developer.blender.org/D12666