Before this change, we simply fail if the device does not expose one
queue family that supports both graphics and present. Currently this is
fine, since devices tend to lay out their queues in this way. NV, for
instance, tends to have one queue family for all graphics operations and
one more for transfer only. However, it's not a hard requirement, and it
is cheap to use a separate queue, so we might as well.
anv seems to set this to zero, which is fine according to the spec, but
we were using it as a maximum, which was resulting in a swap chain
without any buffers being created.
