vulkan: Don't create more than one render object

vulkan: Don't create more than one render object

By creating unlimited render objects, we would never wait on the GPU.

This would mean that if the GPU was the bottleneck, we would fill its
queue with render commands faster than it could process them.

And because the nvidia binary driver and my code work surprisingly well
and bugfree, this lead to exhaustion of RAM. I had 50GB of swap
configured and my hard disk was quicker as swap storage than my GPU was
at processing the commands, so stuff still filled up.

At that point my computer became rather unresponsive and I decided to
reboot it, so I that could write this patch.