Depsgraph: cache effector relations, for performance and stability.

To find all effectors in the scene, we need to loop over all objects.
Doing this during depsgraph evaluation caused crashes because not all
objects are guaranteed to be evaluated yet.

To fix this, we now cache the relations as part of the dependency graph
build. As a bonus this also makes evaluation faster for big scenes,
since looping over all objects for each particle system is slow.

Fixes T55156.

source/blender/physics/intern/BPH_mass_spring.cpp

@@ -488,7 +488,7 @@ static void cloth_calc_force(Scene *scene, ClothModifierData *clmd, float UNUSED
 
 			BPH_mass_spring_get_motion_state(data, i, x, v);
 			pd_point_from_loc(scene, x, v, i, &epoint);
-			pdDoEffectors(effectors, NULL, clmd->sim_parms->effector_weights, &epoint, winvec[i], NULL);
+			BKE_effectors_apply(effectors, NULL, clmd->sim_parms->effector_weights, &epoint, winvec[i], NULL);
 		}
 
 		for (i = 0; i < cloth->tri_num; i++) {