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.