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.

release/scripts/modules/bpy_extras/node_shader_utils.py

@@ -486,6 +486,7 @@ class PrincipledBSDFWrapper(ShaderWrapper):
         return ShaderImageTextureWrapper(
             self, self.node_principled_bsdf,
             self.node_principled_bsdf.inputs["Alpha"],
+            use_alpha=True,
             grid_row_diff=-1,
             colorspace_name='Non-Color',
         )