Pass stores draw commands rather than sending them to Vulkan
immediately.
The main motivation is to allow more flexibility in the Lua API. Passes
are now regular objects, aren't invalidated whenever submit is called,
and can cache their draws across multiple frames. Draws can also be
internally culled, sorted, and batched.
Some API methods (tallies) are missing, and there are still some bugs to
fix, notably with background color.
