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-config/src/lib.rs

@@ -23,7 +23,8 @@ use smithay::input::keyboard::xkb::{keysym_from_name, KEYSYM_CASE_INSENSITIVE};
 use smithay::input::keyboard::{Keysym, XkbConfig};
 use smithay::reexports::input;
 
-pub const DEFAULT_BACKGROUND_COLOR: Color = Color::from_array_unpremul([0.2, 0.2, 0.2, 1.]);
+pub const DEFAULT_BACKGROUND_COLOR: Color = Color::from_array_unpremul([0.25, 0.25, 0.25, 1.]);
+pub const DEFAULT_BACKDROP_COLOR: Color = Color::from_array_unpremul([0.15, 0.15, 0.15, 1.]);
 
 pub mod layer_rule;
 
@@ -444,6 +445,8 @@ pub struct Output {
     pub focus_at_startup: bool,
     #[knuffel(child, default = DEFAULT_BACKGROUND_COLOR)]
     pub background_color: Color,
+    #[knuffel(child, default = DEFAULT_BACKDROP_COLOR)]
+    pub backdrop_color: Color,
 }
 
 impl Output {
@@ -472,6 +475,7 @@ impl Default for Output {
             mode: None,
             variable_refresh_rate: None,
             background_color: DEFAULT_BACKGROUND_COLOR,
+            backdrop_color: DEFAULT_BACKDROP_COLOR,
         }
     }
 }
@@ -4127,6 +4131,12 @@ mod tests {
                             b: 0.4,
                             a: 1.0,
                         },
+                        backdrop_color: Color {
+                            r: 0.15,
+                            g: 0.15,
+                            b: 0.15,
+                            a: 1.0,
+                        },
                     },
                 ],
             ),