Function interp_down_viewport 

fn interp_down_viewport(
    args: &[RValue],
    named: &[(String, RValue)],
    context: &BuiltinContext<'_>,
) -> Result<RValue, RError>

Navigate down to a named viewport in the stack.

@param name the name of the viewport to navigate to @return the depth navigated (integer), or error if not found