Render cursor in multi-screen setup in QPainter Compositor

Summary:
The call to render the cursor was missing in the multi-screen code path
of the QPainter Compositor.

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D2027

scene_qpainter.cpp

@@ -163,6 +163,7 @@ qint64 SceneQPainter::paint(QRegion damage, ToplevelList toplevels)
             QRegion updateRegion, validRegion;
             paintScreen(&mask, damage.intersected(geometry), QRegion(), &updateRegion, &validRegion);
             overallUpdate = overallUpdate.united(updateRegion);
+            m_backend->renderCursor(m_painter.data());
 
             m_painter->restore();
             m_painter->end();