Blender 3.4 moved material indices to a generic attribute. The old API
still works for now, but is slower and may be removed in 4.0, so this
patch updates FBX IO to use the new "material_index" attribute.
This patch changes the export of meshes that have multiple materials,
but where all the polygons are assigned to the first material (so there
is no "material_index" attribute) to export using the "AllSame" type.
Material indices exported in this way export in negligible constant
time.
