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).
The idea here is that we can do more complex combinations and use that
to support texture-scale nodes or use fancy texture formats (suc as
YUV).
I'm not sure this is actually necessary, but for now it gives more
flexibility.
Instead of having a descriptor set per operation, we just have one
descriptor set and bind all our images into it.
Then the shaders get to use an index into the large texture array
instead.
Getting this to work - because it's a Vulkan extension that needs to be
manually enabled, even though it's officially part of Vulkan 1.2 - is
insane.
