a449625fc4
This makes the `material_index` attribute built-in on `CurvesGeometry` which means that it's built-in on `Curves` and `GreasePencil`. While only Grease Pencil can use it so far during rendering, we already depend on them on curves when converting between curves and Grease Pencil. Making the attribute built-in implies that it is locked to one domain and type (`Curve` domain and `int32` type in this case). This makes curves more consistent with meshes. Pull Request: https://projects.blender.org/blender/blender/pulls/133488