Fix T69573: Driver Value does not slide along the actual value in the Driver Editor

This change is two-fold:

- Ensure the result of the F-Curve evaluation is stored on the FCurve
  object. This was done in 2.79 but lost when we moved to more granular
  per-curve evaluation from the depsgraph.
- Flush this result from the CoW copy back to the original.

Reviewed by: sergey

Differential Revision: https://developer.blender.org/D5888

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

@@ -900,19 +900,17 @@ void DepsgraphNodeBuilder::build_driver(ID *id, FCurve *fcurve, int driver_index
 {
   /* Create data node for this driver */
   ID *id_cow = get_cow_id(id);
-  ChannelDriver *driver_orig = fcurve->driver;
 
   /* TODO(sergey): ideally we could pass the COW of fcu, but since it
    * has not yet been allocated at this point we can't. As a workaround
    * the animation systems allocates an array so we can do a fast lookup
    * with the driver index. */
-  ensure_operation_node(
-      id,
-      NodeType::PARAMETERS,
-      OperationCode::DRIVER,
-      function_bind(BKE_animsys_eval_driver, _1, id_cow, driver_index, driver_orig),
-      fcurve->rna_path ? fcurve->rna_path : "",
-      fcurve->array_index);
+  ensure_operation_node(id,
+                        NodeType::PARAMETERS,
+                        OperationCode::DRIVER,
+                        function_bind(BKE_animsys_eval_driver, _1, id_cow, driver_index, fcurve),
+                        fcurve->rna_path ? fcurve->rna_path : "",
+                        fcurve->array_index);
   build_driver_variables(id, fcurve);
 }