This refactors how a geometry nodes node tree is converted to a lazy-function
graph. Previously, all nodes were inserted into a single graph. This was fine
because every node was evaluated at most once per node group evaluation.
However, loops (#108896) break this assumption since now nodes may be
evaluated multiple times and thus a single flat graph does not work anymore.
Now, a separate lazy-function is build for every zone which gives us much
more flexibility for what can happen in a zone. Right now, the change only
applies to simulation zones since that's the only kind of zone we have.
Technically, those zones could be inlined, but turning them into a separate
lazy-function also does not hurt and makes it possible to test this refactor
without implementing loops first. Also, having them as separate functions
might help in the future if we integrate a substep loop directly into the
simulation zone.
The most tricky part here is to just link everything up correctly, especially
with respect to deterministic anonymous attribute lifetimes. Fortunately,
correctness can be checked visually by looking at the generated graphs.
The logging/viewer system also had to be refactored a bit, because now there
can be multiple different `ComputeContext` in a single node tree. Each zone
is in a separate `ComputeContext`. To make it work, the `ViewerPath` system
now explicitly supports zones and drawing code will look up the right logger
for showing inspection data.
No functional changes are expected, except that the spreadsheet now shows
"Simulation Zone" in the context path if the viewer is in a simulation.
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