vulkan/wsi: Set X11 minImageCount to 3.

vulkan/wsi: Set X11 minImageCount to 3.

For IMMEDIATE and FIFO, most games work in a pipelined manner where the
can produce frames at a rate of 1/MAX(CPU duration, GPU duration), but
the render latency is CPU duration + GPU duration.

This means that with scanout from pageflipping we need 3 frames to run
full speed:
1) CPU rendering work
2) GPU rendering work
3) scanout

Once we have a nonblocking acquire that returns a semaphore we can merge
1 and 3. Hence the ideal implementation needs only 2 images, but games
cannot tellwe currently do not have an ideal implementation and that
hence they need to allocate 3 images. So let us do it for them.

This is a tradeoff as it uses more memory than needed for non-fullscreen
and non-performance intensive applications.

Since this is pretty much a TODO that can use the context I added this as
a comment.

Acked-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>