Fixes a segfault on Mac when resizing the window by doing two innocuous but otherwise "good" things:

1. Provide the old swapchain when recreating a new swapchain
2. Wait for a successful present before destroying the old swapchain

The issue was easy to reproduce by taking any example and dragging the window around; after 5-10s it would happen.

Also removes the vk::Result::SUBOPTIMAL_KHR error case during acquire_next_image() because ash never returns it.