Blender Git Loki

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