Move background rendering to Workspace

Per-output background-color doesn't work yet.

niri-visual-tests/src/cases/layout.rs

@@ -272,7 +272,7 @@ impl TestCase for Layout {
             .monitor_for_output(&self.output)
             .unwrap()
             .render_elements(renderer, RenderTarget::Output, true)
-            .flat_map(|(_, iter)| iter)
+            .flat_map(|(_, _, iter)| iter)
             .map(|elem| Box::new(elem) as _)
             .collect()
     }