Vulkan/X11/WSI: Handle resizes more gracefully in general

Three commits, each with their own justification. This fixes at least one race condition I can (very infrequently) reproduce with mpv, and also makes "live resizing" much smoother in general. In particular, the window contents no longer appears "visually frozen" while resizing, since mpv now no longer has to spam so many swapchain recreations that it fails actually presenting to any of these.