vulkan/swapchain: explicitly wait for queues to be idle

This is technically undefined behavior otherwise, at least until there's an official resolution upstream. So, use this as safety precaution for now. In theory this should be a no-op because we already emit vk_wait_idle before that point.

See-Also: https://github.com/KhronosGroup/Vulkan-Docs/issues/1678 See-Also: https://github.com/KhronosGroup/Vulkan-ValidationLayers/issues/4733