On Wayland, make wl_subcompositor protocol optional

This protocol is only used for (optional) Client Side Decorations
(where) the compositor still takes the burden of compositing various
window parts together, via subsurfaces that all belong to a single
window.

If this core protocol is not available, as is the case on gamescope,
disable CSD.

src/platform_impl/linux/wayland/state.rs

@@ -50,7 +50,7 @@ pub struct WinitState {
     pub compositor_state: Arc<CompositorState>,
 
     /// The state of the subcompositor.
-    pub subcompositor_state: Arc<SubcompositorState>,
+    pub subcompositor_state: Option<Arc<SubcompositorState>>,
 
     /// The seat state responsible for all sorts of input.
     pub seat_state: SeatState,
@@ -124,12 +124,17 @@ impl WinitState {
         let registry_state = RegistryState::new(globals);
         let compositor_state =
             CompositorState::bind(globals, queue_handle).map_err(WaylandError::Bind)?;
-        let subcompositor_state = SubcompositorState::bind(
+        let subcompositor_state = match SubcompositorState::bind(
             compositor_state.wl_compositor().clone(),
             globals,
             queue_handle,
-        )
-        .map_err(WaylandError::Bind)?;
+        ) {
+            Ok(c) => Some(c),
+            Err(e) => {
+                warn!("Subcompositor protocol not available, ignoring CSD: {e:?}");
+                None
+            }
+        };
 
         let output_state = OutputState::new(globals, queue_handle);
         let monitors = output_state.outputs().map(MonitorHandle::new).collect();
@@ -151,7 +156,7 @@ impl WinitState {
         Ok(Self {
             registry_state,
             compositor_state: Arc::new(compositor_state),
-            subcompositor_state: Arc::new(subcompositor_state),
+            subcompositor_state: subcompositor_state.map(Arc::new),
             output_state,
             seat_state,
             shm: Shm::bind(globals, queue_handle).map_err(WaylandError::Bind)?,

src/platform_impl/linux/wayland/window/state.rs

@@ -254,7 +254,7 @@ impl WindowState {
         &mut self,
         configure: WindowConfigure,
         shm: &Shm,
-        subcompositor: &Arc<SubcompositorState>,
+        subcompositor: &Option<Arc<SubcompositorState>>,
         event_sink: &mut EventSink,
     ) -> LogicalSize<u32> {
         // NOTE: when using fractional scaling or wl_compositor@v6 the scaling
@@ -265,10 +265,11 @@ impl WindowState {
             self.stateless_size = self.size;
         }
 
-        if configure.decoration_mode == DecorationMode::Client
-            && self.frame.is_none()
-            && !self.csd_fails
-        {
+        if let Some(subcompositor) = subcompositor.as_ref().filter(|_| {
+            configure.decoration_mode == DecorationMode::Client
+                && self.frame.is_none()
+                && !self.csd_fails
+        }) {
             match WinitFrame::new(
                 &self.window,
                 shm,