Previously, whenever a node had a non-trivial storage struct, there would have
to be code for it in `node.cc`. Now there is a general callback that new node
types can use to implement their blend read/write behavior.
Some existing nodes were converted to use this decentralized method. However,
some older nodes can't use it in the same way, because the node types were
introduced before there were node idnames. It's also somewhat hard to reason
about special cases that versioning code might have for these nodes, so they
remain unchanged.
The node callback only writes the non-trivial data, while the main node storage
struct is written automatically by relying on `bNodeType::storagename`. This
simplifies the callback in many cases or makes it unnecessary for trivial types.
Some nodes have specific handling for forward-compatibility. This
forward-compatibility code is kept in `node.cc` for now, because it also affects
the main storage struct and therefore has to be changed before that struct is
written.
Pull Request: https://projects.blender.org/blender/blender/pulls/138722
23d5ae7da5