Blender Git Loki

Fix T80684: Node shader wrapper: Tweak handling of alpha textures.

There is no ideal solution here, but we can assume by default that we should
use the Alpha output from a texture used for transparency data.

This will break people using a dedicated B&W texture for this alpha
pass, but we cannot really handle all cases properly in this wrapper, we
only try to support the most common ones to some extent.