Implement place-within-backdrop layer rule

niri-config/src/layer_rule.rs

@@ -15,6 +15,8 @@ pub struct LayerRule {
     pub shadow: ShadowRule,
     #[knuffel(child)]
     pub geometry_corner_radius: Option<CornerRadius>,
+    #[knuffel(child, unwrap(argument))]
+    pub place_within_backdrop: Option<bool>,
 }
 
 #[derive(knuffel::Decode, Debug, Default, Clone, PartialEq)]

niri-config/src/lib.rs

@@ -4888,6 +4888,7 @@ mod tests {
                         inactive_color: None,
                     },
                     geometry_corner_radius: None,
+                    place_within_backdrop: None,
                 },
             ],
             binds: Binds(