Fix a crash in ImporterMesh::create_shadow_mesh for non-triangle surfaces

[zeux]

optimize_vertex_cache_func assumes the input is a triangle mesh; when an imported mesh contains points/lines/triangle strips, this code should be disabled.

Introduced in #94241
Fixes #96869

[zeux]

Aside: in the linked issue report, the primitives in question are lines; creating shadow meshes for point/line primitives seems dubious. However, it's still valuable for triangle strips in case they get through here through the importer flow, so this check is probably still a good idea even if we end up adding some early outs for lines/points in the future.

[clayjohn]

Makes sense! Indeed creating a shadow mesh should be skipped anyway for points and line primitives, I can't see how it would be beneficial

[akien-mga]

Thanks!