Function parse_obj_with_resolver 

pub fn parse_obj_with_resolver<R>(text: &str, resolve: R) -> Result<Scene3D>
where
    R: FnMut(&str) -> Result<Vec<u8>>,

Parse an OBJ document, calling resolve once per mtllib entry to fetch the bytes of the named material library. Each library is parsed via parse_mtl and its materials merged into the resulting scene; references in usemtl directives bind to those materials by name.

The resolver returns Ok(Vec::new()) to signal “this library couldn’t be located but skip silently”; any other Err aborts the parse.