Remove layer from superlayer when dropping (#326)

Signed-off-by: Wu Wayne <yuweiwu@pm.me>

src/platform/macos/system/surface.rs

@@ -487,6 +487,8 @@ impl Drop for ViewInfo {
 
             // Drop the reference that the callback was holding onto.
             let _ = mem::transmute_copy::<Arc<VblankCond>, Arc<VblankCond>>(&self.next_vblank);
+
+            self.layer.remove_from_superlayer();
         }
     }
 }