Function ltn::draw_with_layering

pub(crate) fn draw_with_layering<F: Fn(&mut GfxCtx<'_>)>(
    g: &mut GfxCtx<'_>, 
    app: &SimpleApp<Session>, 
    custom: F
)

Do the equivalent of SimpleApp::draw_unzoomed or draw_zoomed, but after the water/park areas layer, draw something custom.