`bNodeTree` has a lot of run-time embedded in it currently. Having a separately
allocated run-time struct has some benefits:
* Run-time data is not stored in files.
* Makes it easy to use c++ types as run-time data.
* More clear distinction between what data only exists at run-time and which doesn't.
This commit doesn't move all run-time data to the new struct yet, only the data where
I know for sure how it is used. The remaining data can be moved separately.
Differential Revision: https://developer.blender.org/D15033
bb0fc67582