Introduced in 853269aeb0
Prior to this commit, the PBVH partitioning process did not work
correctly for multires meshes with materials. Specifically, it failed
upon mapping the partitioned faces into their corresponding corners.
The rough process here is as follows:
* Flatten out the array of face indices into an array of corner indices.
* Sum up each `GridsNode` `prim_indices` corner count into an array.
* Create an `OffsetIndices` from these sums
* Use the `OffsetIndices` to slice the array created at the beginning
to assign to each node.
However, this process requires that the main PBVH array of corner
indices has the same order as iterating over the nodes, which the
partitioning algorithm does not do.
To solve this, this commit iterates over the Node `prim_indices` `Span`s
in the same order that the nodes are stored when flatting out the
corner data, ensuring a correct mapping.
Pull Request: https://projects.blender.org/blender/blender/pulls/129392
1dd6cdb172