Render workspaces separately with gaps between

This design makes more sense spatially, and is required for the
Overview. Gaps also make it clear how clipping windows to workspace
bounds works.

Background and bottom layer-shell surfaces get duplicated for each
workspace, while top and overlay stay "on top".

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

@@ -266,6 +266,7 @@ impl TestCase for Layout {
             .monitor_for_output(&self.output)
             .unwrap()
             .render_elements(renderer, RenderTarget::Output, true)
+            .flat_map(|(_, iter)| iter)
             .map(|elem| Box::new(elem) as _)
             .collect()
     }