subsurface_widget: Clear subsurface list after calls to ::draw()

Otherwise the `draw` call in response to `AboutToWait` causes this list
to grow, and eventually hit errors in `sendmsg` due to too many buffered
file descriptors from creating subsurfaces.

Probably the number of places `UserInterface::draw` are called should be
cleaned up. And it shouldn't be called in `AboutToWiat` like this. Some
improvements could probably be made in upstream iced.

winit/src/lib.rs

@@ -1133,6 +1133,7 @@ async fn run_instance<P>(
                     },
                     cursor,
                 );
+                platform_specific_handler.clear_subsurface_list();
                 draw_span.finish();
 
                 if let user_interface::State::Updated {

winit/src/platform_specific/mod.rs

@@ -72,6 +72,13 @@ impl PlatformSpecific {
         }
     }
 
+    pub(crate) fn clear_subsurface_list(&mut self) {
+        #[cfg(all(feature = "wayland", target_os = "linux"))]
+        {
+            self.wayland.clear_subsurface_list();
+        }
+    }
+
     pub(crate) fn update_subsurfaces(
         &mut self,
         id: window::Id,

winit/src/platform_specific/wayland/mod.rs

@@ -181,6 +181,10 @@ impl WaylandSpecific {
         };
     }
 
+    pub(crate) fn clear_subsurface_list(&mut self) {
+        let _ = crate::subsurface_widget::take_subsurfaces();
+    }
+
     pub(crate) fn update_subsurfaces(
         &mut self,
         id: window::Id,