Port to asynchronous roundtrip

When a resize is driven client side we wait for the compositor to have a
chance to reconfigure us before submitting the next frame.

Using a blocking round trip caused an issue. Instead block isExposed and
trigger an expose event whilst a sync is in progress.

src/qwaylandlayersurface_p.h

@@ -30,7 +30,7 @@ public:
 
     bool isExposed() const override
     {
-        return m_configured;
+        return m_configured && !m_waitForSyncCallback;
     }
     void attachPopup(QtWaylandClient::QWaylandShellSurface *popup) override;
 
@@ -49,6 +49,9 @@ public:
     void requestXdgActivationToken(quint32 serial) override;
 
 private:
+    void requestWaylandSync();
+    void handleWaylandSyncDone();
+    void sendExpose();
     void zwlr_layer_surface_v1_configure(uint32_t serial, uint32_t width, uint32_t height) override;
     void zwlr_layer_surface_v1_closed() override;
 
@@ -57,7 +60,11 @@ private:
     QtWaylandClient::QWaylandWindow *m_window;
     QSize m_pendingSize;
     QString m_activationToken;
+
     bool m_configured = false;
+
+    static const wl_callback_listener syncCallbackListener;
+    struct wl_callback *m_waitForSyncCallback = nullptr;
 };
 
 }