This was motivated by experiments with the Vulkan memory model. To use
that, we actually need to explicitly enable the relevant feature on
device creation time. That's a lot easier to do now that push_next works
on the structs in that chain. This PR doesn't do that though, it only
upgrades the dependency and cleans up deprecations.
Make the scene dependent on timing.
This commit patches the HAL to reuse command buffers; this works well on
Vulkan and prevents a leak, but breaks the other back-ends. That will
require a solution, possibly including plumbing up the resource lifetime
responsibilities to the client.
Other things might be hacky as well.
