Cycles/Eeeve: unify film transparent setting

For existing files, it will use the setting from Cycles or Eevee depending on
the render engine in the scene.

Differential Revision: https://developer.blender.org/D4874

release/scripts/modules/bl_previews_utils/bl_previews_render.py

@@ -123,7 +123,7 @@ def do_previews(do_objects, do_collections, do_scenes, do_data_intern):
             scene.collection.objects.link(light)
 
             scene.render.engine = 'CYCLES'
-            scene.cycles.film_transparent = True
+            scene.render.film_transparent = True
             # TODO: define Cycles world?
 
         scene.render.image_settings.file_format = 'PNG'