This function was incorrectly using the surface number to index into the
LOD indices vector. This resulted in just returning the same index over
and over again. In theory if you had a mesh with more surfaces than one
of its LOD vectors it could read pass the end of the LOD index array.
The SoftBody3D code creates a new ArrayMesh by duplicating the input
mesh, and uses `mesh_surface_get_lods()` to duplicate the LODs. The
broken behavior here results in SoftBody3D creating broken meshes that
render nothing due to each LOD just using a single vertex. This commit
fixes SoftBody3D to now work correctly with meshes with LODs.
Fixes#107984.
ad4dff7cbe