Previously, the geometry nodes modifier was converting the
viewer path to a compute context at the same time as it was
setting up side effect nodes for the geometry nodes evaluation.
Now, this is changed to be a two step process. First, the viewer
path is converted to the corresponding compute context.
Afterwards, a separate function sets side effect nodes up so
that the given node in the given compute context will be evaluated.
This has three main benefits:
* More obvious separation of concerns.
* Can reuse the code that maps a viewer path element to a compute
context already.
* With gizmo nodes (#112677), it may become necessary to add side
effect nodes based on a compute context, but without having a
corresponding viewer path.
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