Wayland: XComposite backend does not update surfaces properly

When using QT_WAYLAND_CLIENT_BUFFER_INTEGRATION=xcomposite-egl, client windows are not updated when they redraw. If the compositor redraws for some other reason (e.g. if the cursor moves, or a shmBuffer client is animating), then the xcomposite application will also be updated.

This happens because QWaylandWindow::commit() does not allow the same buffer to be committed multiple times. The XComposite backend uses only one QWaylandBuffer for each surface, since the actual communication is done through the XComposite extension. This works similarly to Nvidia eglStreams, but in that case the commit is performed by the graphics driver, and not by Qt.