As discussed in T38340 the solution is to use the current scene from
context whenever feasible.
Composite does not use node->id at all now, the scene which owns the
compositing node tree is retrieved from context instead.
Defocus node->id is made editable by the user. By default it is not set,
which also will make it use the contextual scene and camera info.
The node->id pointer in Defocus is **not** cleared in older blend files.
This is done for backward compatibility: the node will then behave as
before in untouched scenes.
File Output nodes also don't store scene in node->id. This is only needed
when creating a new node for initializing the file format.
Reviewers: brecht, jbakker, mdewanchand
Reviewed By: brecht
Differential Revision: https://developer.blender.org/D290
1687023776