vulkan: Reserve 4 images in the swapchain by default

vulkan: Reserve 4 images in the swapchain by default

With the previous approach we would spend most of the time waiting for
the swapchain to be filled again because it seems the compositor takes
care of 2 images at once from time to time.

This is not visible in profiles because waiting for a frame is a
read/poll/whatever operation that does not take CPU. It's only
noticeable because the app becomes less responsive.