[kwin_wayland] Flush display connection before going to block

Integrate with QAbstractEventDispatcher::aboutToBlock and because of that
we can drop all direct calls to flush from the Scenes.

egl_wayland_backend.cpp

@@ -250,9 +250,6 @@ bool EglWaylandBackend::initBufferConfigs()
 
 void EglWaylandBackend::present()
 {
-    // need to dispatch pending events as eglSwapBuffers can block
-    m_wayland->dispatchEvents();
-
     m_lastFrameRendered = false;
     wl_callback *callback = wl_surface_frame(m_wayland->surface());
     wl_callback_add_listener(callback, &s_surfaceFrameListener, this);

scene_qpainter.cpp

@@ -140,7 +140,6 @@ void WaylandQPainterBackend::present(int mask, const QRegion &damage)
         wl_surface_damage(surface, rect.x(), rect.y(), rect.width(), rect.height());
     }
     wl_surface_commit(surface);
-    wl->dispatchEvents();
 }
 
 void WaylandQPainterBackend::lastFrameRendered()

scene_xrender.cpp

@@ -349,7 +349,6 @@ void WaylandXRenderBackend::present(int mask, const QRegion &damage)
         wl_surface_damage(surface, rect.x(), rect.y(), rect.width(), rect.height());
     }
     wl_surface_commit(surface);
-    wl->dispatchEvents();
 }
 
 bool WaylandXRenderBackend::isLastFrameRendered() const

wayland_backend.cpp

@@ -28,6 +28,8 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #include "wayland_client/registry.h"
 #include "wayland_client/shell.h"
 // Qt
+#include <QAbstractEventDispatcher>
+#include <QCoreApplication>
 #include <QDebug>
 #include <QImage>
 #include <QTemporaryFile>
@@ -571,6 +573,15 @@ WaylandBackend::WaylandBackend(QObject *parent)
     , m_connectionThread(nullptr)
     , m_fullscreenShell(new FullscreenShell(this))
 {
+    QAbstractEventDispatcher *dispatcher = QCoreApplication::instance()->eventDispatcher();
+    connect(dispatcher, &QAbstractEventDispatcher::aboutToBlock, this,
+        [this]() {
+            if (!m_display) {
+                return;
+            }
+            wl_display_flush(m_display);
+        }
+    );
     connect(this, &WaylandBackend::shellSurfaceSizeChanged, this, &WaylandBackend::checkBackendReady);
     connect(m_registry, &Registry::compositorAnnounced, this,
         [this](quint32 name) {
@@ -643,7 +654,6 @@ void WaylandBackend::initConnection()
             m_registry->create(m_display);
             wl_proxy_set_queue((wl_proxy*)m_registry->registry(), m_eventQueue);
             m_registry->setup();
-            wl_display_flush(m_display);
         },
         Qt::QueuedConnection);
     connect(m_connectionThreadObject, &ConnectionThread::eventsRead, this,
@@ -757,12 +767,6 @@ void WaylandBackend::addOutput(wl_output *o)
     connect(output, &Output::changed, this, &WaylandBackend::outputsChanged);
 }
 
-void WaylandBackend::dispatchEvents()
-{
-    // TODO: integrate into event loop to flush before going to block
-    wl_display_flush(m_display);
-}
-
 wl_registry *WaylandBackend::registry()
 {
     return m_registry->registry();

wayland_backend.h

@@ -190,8 +190,6 @@ public:
     wl_surface *surface() const;
     QSize shellSurfaceSize() const;
     void installCursorImage(Qt::CursorShape shape);
-
-    void dispatchEvents();
 Q_SIGNALS:
     void shellSurfaceSizeChanged(const QSize &size);
     void systemCompositorDied();