Client: Don't always recreate frame callbacks

The main QWaylandWindow method that is executed when handling updates is QWaylandWindow::handleUpdate(). This method always, unconditionally queues a frame callback, regardless of whether any other one is already queued.

On some circumstances, e.g. when a window is hidden or completely obscured by other windows, it stops receiving frame callbacks from the compositor. However, QWaylandWindow would continue to request for them, which eventually fills up the Wayland socket, and causes the application to crash.

This can be avoided by checking if the platform window is already waiting for a frame callback, before queueing another one.

In QWaylandWindow::handleUpdate(), check if mWaitingForFrameCallback is true before queueing frame callbacks, and early return if that's the case.

The XDG-shell test needed to be updated for this: The mock compositor is not responding to any frame callbacks, so the window will be unexposed, no longer get paint events and therefore not trigger any commit. This worked by accident before because we were issuing updates quickly enough to reset the timer before it had a chance to unexpose the window. The easiest fix is just to disable the dependency on frame callbacks in this test, since that is clearly not what it's testing.

Task-number: QTBUG-81504 Change-Id: Ieacb05c7d5a5fcf662243d9177ebcc308cb9ca84