Xwayland: some Vulkan apps only get 58fps in windowed mode

It randomly happens that vkcube only gets 58 fps. These 58fps come from the copy timer: https://gitlab.freedesktop.org/xorg/xserver/-/blob/master/hw/xwayland/xwayland-present.c#L47

When vkcube is resized, there's a chance that it begins working correctly, or that it falls back to the timer again. Making vkcube fullscreen and going back to windowed mode fixes things, at least until it's resized again.

This only happens on high refresh rate monitors - at 60Hz vkcube always uses the correct refresh rate.