Fix T80121: Forcefield F-curve modifier changes don't reset cache

Add a dependency graph relation Force Object Animation → Scene Rigid
Body World Rebuild. This ensures that the rigid body world is rebuilt
when a force object is re-tagged for animation updates.

The extra relation doesn't add any new calculations when the animation
is running, as the Time Source node already had a relation to the
scene's `RIGIDBODY_REBUILD` node.

The relation is created directly to the `RIGIDBODY_REBUILD` Operation. I
would have liked to target the containing Component instead. However,
that has the `RIGIDBODY_SIM` operation as entry node, which isn't enough
to actually fix T80121.

Reviewers: Sergey

Differential Revision: https://developer.blender.org/T80121

source/blender/depsgraph/intern/builder/deg_builder_relations.cc

@@ -1252,6 +1252,7 @@ void DepsgraphRelationBuilder::build_animdata(ID *id)
     ComponentKey animation_key(id, NodeType::ANIMATION);
     ComponentKey parameters_key(id, NodeType::PARAMETERS);
     add_relation(animation_key, parameters_key, "Animation -> Parameters");
+    build_animdata_force(id);
   }
 }
 
@@ -1396,6 +1397,24 @@ void DepsgraphRelationBuilder::build_animation_images(ID *id)
   }
 }
 
+void DepsgraphRelationBuilder::build_animdata_force(ID *id)
+{
+  if (GS(id->name) != ID_OB) {
+    return;
+  }
+
+  const Object *object = (Object *)id;
+  if (object->pd == nullptr || object->pd->forcefield == PFIELD_NULL) {
+    return;
+  }
+
+  /* Updates to animation data (in the UI, for example by altering FCurve Modifier parameters
+   * animating force field strength) may need to rebuild the rigid body world. */
+  ComponentKey animation_key(id, NodeType::ANIMATION);
+  OperationKey rigidbody_key(&scene_->id, NodeType::TRANSFORM, OperationCode::RIGIDBODY_REBUILD);
+  add_relation(animation_key, rigidbody_key, "Animation -> Rigid Body");
+}
+
 void DepsgraphRelationBuilder::build_action(bAction *action)
 {
   if (built_map_.checkIsBuiltAndTag(action)) {

source/blender/depsgraph/intern/builder/deg_builder_relations.h

@@ -241,6 +241,7 @@ class DepsgraphRelationBuilder : public DepsgraphBuilder {
                                                OperationNode *operation_from,
                                                ListBase *strips);
   virtual void build_animdata_drivers(ID *id);
+  virtual void build_animdata_force(ID *id);
   virtual void build_animation_images(ID *id);
   virtual void build_action(bAction *action);
   virtual void build_driver(ID *id, FCurve *fcurve);