[wayland] Unreference the buffer when destroying the WindowPixmap

When we don't use the buffer anymore, we should discard it. Otherwise
we start to leak buffers and that can result in clients dead locking
while waiting for more free buffers.

scene.cpp

@@ -937,6 +937,13 @@ WindowPixmap::~WindowPixmap()
     if (isValid() && !kwinApp()->shouldUseWaylandForCompositing()) {
         xcb_free_pixmap(connection(), m_pixmap);
     }
+#if HAVE_WAYLAND
+    if (m_buffer) {
+        using namespace KWayland::Server;
+        QObject::disconnect(m_buffer.data(), &BufferInterface::aboutToBeDestroyed, m_buffer.data(), &BufferInterface::unref);
+        m_buffer->unref();
+    }
+#endif
 }
 
 void WindowPixmap::create()