Fail gamma control on output removal

src/backend/udev.rs

@@ -1186,6 +1186,7 @@ impl State {
                 },
             );
             self.space.unmap_output(&output);
+            self.gamma_control_manager_state.output_removed(&output);
         }
     }
 

src/protocol/gamma_control.rs

@@ -43,6 +43,12 @@ impl GammaControlManagerState {
             gamma_controls: HashMap::new(),
         }
     }
+
+    pub fn output_removed(&mut self, output: &Output) {
+        if let Some(gamma_control) = self.gamma_controls.remove(output) {
+            gamma_control.failed();
+        }
+    }
 }
 
 pub struct GammaControlState {