Currently, sometimes shader code generation fails because the code of top-level
nodes is interleaved with the code of a repeat zone. See #147747 for more
details.
Previously, the code-generation order was just the standard toposort of the node
tree, but it does not guarantee that all nodes in a zone are next to each other.
Also, it can't easily, because the zone detection itself depends on the
toposort. This patch implements another sorting step on top of the default
toposort. It packs nodes in zones together without changing the validity of the
toposort.
This is an alternative to #147999.
Pull Request: https://projects.blender.org/blender/blender/pulls/148005
5be95161a4