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/platform_specific/mod.rs

@@ -59,6 +59,13 @@ 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,

winit/src/program.rs

@@ -963,6 +963,8 @@ async fn run_instance<'a, P, C>(
                                     },
                                     Default::default(),
                                 );
+                                platform_specific_handler
+                                    .clear_subsurface_list();
                                 let mut bytes = compositor.screenshot(
                                     &mut renderer,
                                     &mut surface,
@@ -1713,6 +1715,7 @@ async fn run_instance<'a, P, C>(
                             cursor,
                         )
                     };
+                    platform_specific_handler.clear_subsurface_list();
 
                     if new_mouse_interaction != window.mouse_interaction {
                         if let Some(interaction) =