Blender Git Loki

Geometry Nodes: Expose material index attribute

The `material_index` attribute can adjust which material in the list
will be applied to each face of the mesh. There are two new things
about this attribute that haven't been exposed by the attribute API yet.
Each comes with limitations:
1. Integer data type: Most attribute nodes are currently written to use
float data types. This means that they can't write to this attribute
because they can't change the type of a built-in attribute.
2. Polygon domain: This is our first attribute using the polygon domain,
meaning until some of the interpolations are implemented, some
operations may not work as expected.

Currently the two nodes that work with this attribute are Attribute Fill
and Attribute Randomize.

Differential Revision: https://developer.blender.org/D10444