Introduce skid_steer as a new way to load assets

[patowen]

This is a relatively significant refactor of Hypermine, switching up asset loading to use a new library called "skid_steer".

It introduces the following changes:

• A custom AssetLoader type is added to drive asset loading, acting as the glue between skid_steer and Hypermine. This completely replaces the old loader module.
• All uses of the staging buffer and transfer module from the temporary lahar_deprecated module have been replaced with a vendored growable_ring implementation from @Ralith, and a ParallelQueue from the current version of lahar. This allows us to finally remove the lahar_deprecated module, a task that I had hoped to do years ago.
• PNG loading and GLTF loading, the only two processes that depended on the old Loader, have been migrated to the new AssetLoader.
• File-processing (reading PNGs and GLBs) and Vulkan-wrangling (making Vulkan calls) have been fully separated, with all data being read into memory in a convenient form before being passed into Vulkan data structures.
• To support some of the above changes, some data has been moved around, including the addition of a new ShaderData struct to pass around global data used for rendering.

[Ralith]

Review is just getting started, but it may be a few days before I make more progress.

[patowen]

I'll request one more quick review to double-check my latest changes, although based on your previous approval, I'm pretty confident this is safe to merge as-is.