vulkan/wsi: limit content-size to 1x1
It's not legal to create a swapchain with zero width or height in Vulkan. To avoid having to make the code here overly pessimistic, let's just limit the content-size to 1x1 to side-step the problem.
We also do the same for eglut, because things behaves strange there without it. See the second commit for details.
While this might seem like a bit of a hacky work-around, this actually
matches the behavior of glxgears, so