Masked materials on Nanite meshes with multiple material slots causes erroneous cluster culling

When a nanite-enabled mesh with multiple material slots has a masked material applied, nanite may erroneously cull clusters that are still visible.

1. Create a mesh using multiple materials slots via a DCC. I used three. Using a high-poly mesh makes the culling error easier to see.

2. Import the mesh and build nanite.

3, Assigning a masked material will cause clusters to cull erroneously. I used one with a shader constant of 1 assigned to the opacity mask for ease of testing.