Fix T83749: Better handling of alpha in generic Nodes material wrapper for IO add-ons.

Try to detect if a given image may have valid alpha data or not (based
on number of channels and depth). This may be a bit doggy in theory, but
in practice it should cover most fields as expected. We can always
adjust the euristic here in other wrong cases appear.

This will affect all import add-ons using that node shader wrapper (at
least OBJ and FBX ones).

release/scripts/modules/bpy_extras/node_shader_utils.py

@@ -684,6 +684,8 @@ class ShaderImageTextureWrapper():
             self.owner_shader._grid_to_location(-1, 0 + self.grid_row_diff, dst_node=node_image, ref_node=self.node_dst)
 
             tree.links.new(node_image.outputs["Alpha" if self.use_alpha else "Color"], self.socket_dst)
+            if self.use_alpha:
+                self.owner_shader.material.blend_method = 'BLEND'
 
             self._node_image = node_image
         return self._node_image
@@ -703,6 +705,13 @@ class ShaderImageTextureWrapper():
             if image.colorspace_settings.is_data != self.colorspace_is_data and image.users >= 1:
                 image = image.copy()
             image.colorspace_settings.name = self.colorspace_name
+        if self.use_alpha:
+            # Try to be smart, and only use image's alpha output if image actually has alpha data.
+            tree = self.owner_shader.material.node_tree
+            if image.channels < 4 or image.depth in {24, 8}:
+                tree.links.new(self.node_image.outputs["Color"], self.socket_dst)
+            else:
+                tree.links.new(self.node_image.outputs["Alpha"], self.socket_dst)
         self.node_image.image = image
 
     image = property(image_get, image_set)