OBJ IO: Use Principled BSDF's Alpha new setting.

Much better than using Principled's Transmission setting as we did
before...

Part of T64609.

io_scene_obj/__init__.py

@@ -21,7 +21,7 @@
 bl_info = {
     "name": "Wavefront OBJ format",
     "author": "Campbell Barton, Bastien Montagne",
-    "version": (3, 5, 6),
+    "version": (3, 5, 7),
     "blender": (2, 80, 0),
     "location": "File > Import-Export",
     "description": "Import-Export OBJ, Import OBJ mesh, UV's, materials and textures",

io_scene_obj/export_obj.py

@@ -74,7 +74,7 @@ def write_mtl(scene, filepath, path_mode, copy_set, mtl_dict):
 
             if mat_wrap:
                 use_mirror = mat_wrap.metallic != 0.0
-                use_transparency = mat_wrap.transmission != 0.0
+                use_transparency = mat_wrap.alpha != 1.0
 
                 # XXX Totally empirical conversion, trying to adapt it
                 #     (from 1.0 - 0.0 Principled BSDF range to 0.0 - 900.0 OBJ specular exponent range)...
@@ -94,7 +94,7 @@ def write_mtl(scene, filepath, path_mode, copy_set, mtl_dict):
                 # XXX Not supported by current Principled-based shader.
                 fw('Ke 0.0 0.0 0.0\n')
                 fw('Ni %.6f\n' % mat_wrap.ior)  # Refraction index
-                fw('d %.6f\n' % (1.0 - mat_wrap.transmission))  # Alpha (obj uses 'd' for dissolve)
+                fw('d %.6f\n' % mat_wrap.alpha)  # Alpha (obj uses 'd' for dissolve)
 
                 # See http://en.wikipedia.org/wiki/Wavefront_.obj_file for whole list of values...
                 # Note that mapping is rather fuzzy sometimes, trying to do our best here.
@@ -116,7 +116,7 @@ def write_mtl(scene, filepath, path_mode, copy_set, mtl_dict):
                         "map_Ka": None,  # ambient...
                         "map_Ks": "specular_texture",
                         "map_Ns": "roughness_texture",
-                        "map_d": "transmission_texture",
+                        "map_d": "alpha_texture",
                         "map_Tr": None,  # transmission roughness?
                         "map_Bump": "normalmap_texture",
                         "disp": None,  # displacement...

io_scene_obj/import_obj.py

@@ -176,7 +176,7 @@ def create_materials(filepath, relpath,
             _generic_tex_set(mat_wrap.normalmap_texture, image, 'UV', map_offset, map_scale)
 
         elif type == 'D':
-            _generic_tex_set(mat_wrap.transmission_texture, image, 'UV', map_offset, map_scale)
+            _generic_tex_set(mat_wrap.alpha_texture, image, 'UV', map_offset, map_scale)
 
         elif type == 'disp':
             # XXX Not supported?
@@ -245,8 +245,8 @@ def create_materials(filepath, relpath,
             if do_transparency:
                 if "ior" not in context_material_vars:
                     context_mat_wrap.ior = 1.0
-                if "transmission" not in context_material_vars:
-                    context_mat_wrap.transmission = 1.0
+                if "alpha" not in context_material_vars:
+                    context_mat_wrap.alpha = 1.0
                 # EEVEE only
                 context_material.blend_method = 'BLEND'
 
@@ -341,8 +341,8 @@ def create_materials(filepath, relpath,
                         context_mat_wrap.ior = float_func(line_split[1])
                         context_material_vars.add("ior")
                     elif line_id == b'd':  # dissolve (transparency)
-                        context_mat_wrap.transmission = 1.0 - float_func(line_split[1])
-                        context_material_vars.add("transmission")
+                        context_mat_wrap.alpha = float_func(line_split[1])
+                        context_material_vars.add("alpha")
                     elif line_id == b'tr':  # translucency
                         print("WARNING, currently unsupported 'tr' translucency option, skipped.")
                     elif line_id == b'tf':