Fix T80035: Fix crash switching/adding scenes

Ensure that time source always is in the dependency graph, allowing to
tag the graph for time update prior it was fully built.

Collaboration of Philipp Oeser, Jacques Lucke and myself.
Thanks everyone :)

source/blender/depsgraph/intern/depsgraph.cc

@@ -77,6 +77,8 @@ Depsgraph::Depsgraph(Main *bmain, Scene *scene, ViewLayer *view_layer, eEvaluati
   memset(id_type_updated, 0, sizeof(id_type_updated));
   memset(id_type_exist, 0, sizeof(id_type_exist));
   memset(physics_relations, 0, sizeof(physics_relations));
+
+  add_time_source();
 }
 
 Depsgraph::~Depsgraph()