Macro rendy::util::rendy_backend_match

macro_rules! rendy_backend_match {
    (
$ target : path {
$ ( empty => $ empty_code : block ) ? $ ( dx12 => $ dx12_code : block ) ? $ (
metal => $ metal_code : block ) ? $ ( vulkan => $ vulkan_code : block ) ? } ) => { ... };
    ( $ target : path as $ back : ident => $ code : block ) => { ... };
}

Execute arm with matching backend. If particular backend is disabled then its arm is stripped from compilation altogether.