Mesh: Move material indices to a generic attribute

This patch moves material indices from the mesh `MPoly` struct to a
generic integer attribute. The builtin material index was already
exposed in geometry nodes, but this makes it a "proper" attribute
accessible with Python and visible in the "Attributes" panel.

The goals of the refactor are code simplification and memory and
performance improvements, mainly because the attribute doesn't have
to be stored and processed if there are no materials. However, until
4.0, material indices will still be read and written in the old
format, meaning there may be a temporary increase in memory usage.

Further notes:
* Completely removing the `MPoly.mat_nr` after 4.0 may require
  changes to DNA or introducing a new `MPoly` type.
* Geometry nodes regression tests didn't look at material indices,
  so the change reveals a bug in the realize instances node that I fixed.
* Access to material indices from the RNA `MeshPolygon` type is slower
  with this patch. The `material_index` attribute can be used instead.
* Cycles is changed to read from the attribute instead.
* BMesh isn't changed in this patch. Theoretically it could be though,
  to save 2 bytes per face when less than two materials are used.
* Eventually we could use a 16 bit integer attribute type instead.

Ref T95967

Differential Revision: https://developer.blender.org/D15675

1 files changed, 1 insertions, 1 deletions

diff --git a/release/scripts/startup/bl_ui/properties_data_mesh.py b/release/scripts/startup/bl_ui/properties_data_mesh.py
index 8f2141ba6fc..686d455b6b4 100644
--- a/release/scripts/startup/bl_ui/properties_data_mesh.py
+++ b/release/scripts/startup/bl_ui/properties_data_mesh.py
@@ -580,7 +580,7 @@ class DATA_PT_mesh_attributes(MeshButtonsPanel, Panel):
         colliding_names = []
         for collection in (
                 # Built-in names.
-                {"position": None, "material_index": None, "shade_smooth": None, "normal": None, "crease": None},
+                {"position": None, "shade_smooth": None, "normal": None, "crease": None},
                 mesh.attributes,
                 mesh.uv_layers,
                 ob.vertex_groups,