Cycles: disable light tree for existing scenes, enable on new scenes

While it helps on many scenes, it can be disruptive for existing scenes and
for benchmarks the differences in timing can be confusing. So be a bit more
conservative and only it enable it for new scenes.

intern/cycles/blender/addon/version_update.py

@@ -241,6 +241,12 @@ def do_versions(self):
                         layer.samples *= layer.samples
                     cscene["use_square_samples"] = False
 
+            # Disable light tree for existing scenes.
+            if version <= (3, 5, 3):
+                cscene = scene.cycles
+                if not cscene.is_property_set("use_light_tree"):
+                    cscene.use_light_tree = False
+
         # Lamps
         for light in bpy.data.lights:
             if light.library not in libraries: