vulkan: Create multiple render objects

vulkan: Create multiple render objects

Sometimes the GPU is still busy when the next frame starts (like when
no-vsync benchmarking), so we need to keep all those resources alone and
create new ones.
That's what the render object is for, so we just create another one.

However, when we create too many, we'll starve the CPU. So we'll limit
it. Currently, that limit is at 4, but I've never reached it (I've also
not starved the GPU yet), so that number may want to be set lower/higher
in the future.

Note that this is different from the number of outstanding buffers, as
those are not busy on the GPU but on the compositor, and as such a
buffer may have not finished rendering but have been returend from the
compositor (very busy GPU) or have finished rendering but not been
returned from the compositor (very idle GPU).