Alternatively, if we are introducing the whole flush-before-present win32u logic only for the benefit of the Wayland driver, perhaps the simplest way forward would be to provide an (optional) `vulkan_funcs.p_queue_present_done(HWND)` callback and just let each driver do what it needs to do (so NULL for most drivers).
Right, I'll do something like that and leave the other considerations for later.