Client: Always trigger a commit in flush

There are a few paths in wayland that require the client to always
commit to apply changes or ack requests. We need this synchronized with
any rendering so it is normally triggered with an expose or update event
relying that this will always commit at the end.

There is a path in QWidget code where if the window is not damaged we
never call backingStore->beginPaint. If this doesn't happen we don't
swap the front buffer, and the expose event will no-op. We want to avoid
attaching in this case, but we should commit without attaching
regardless to flush these events.

This fixes an issue where Qt could fail to ack configure events which
don't trigger updates.

Pick-to: 6.8
Change-Id: Iaa606b0bf9fbbaefd3e67444cc768f95076967fe
Reviewed-by: Vlad Zahorodnii <vlad.zahorodnii@kde.org>

src/plugins/platforms/wayland/qwaylandwindow.cpp

@@ -740,7 +740,8 @@ void QWaylandWindow::commit(QWaylandBuffer *buffer, const QRegion &damage)
 {
     Q_ASSERT(isExposed());
     if (buffer->committed()) {
-        qCDebug(lcWaylandBackingstore) << "Buffer already committed, ignoring.";
+        mSurface->commit();
+        qCDebug(lcWaylandBackingstore) << "Buffer already committed, not attaching.";
         return;
     }