Function with_mesh_source 

pub fn with_mesh_source<R>(
    world: &World,
    source_entity: Entity,
    f: impl FnOnce(&mut MeshSource, &Scope<'_>) -> R,
) -> R

Run f on the world’s MeshSource, which lives on the source entity.

A shared-world entry point, so the scene’s own &mut World can reborrow it next to the mutable borrow of the source.