Upgrade Hotham renderer

[kanerogers]

Background

This change is the most significant since Hotham was released. In it, we have:

• Completely rewritten the rendering backend to use shared buffers and MultiDrawIndirect
• Performed a large-scale rewrite of the glTF importer
• Written a new abstraction around vk::Buffer

Overall, I'm really happy with the work, and believe this lays down a solid foundation moving forward.

Next steps

• [Feature] Optimise new renderer #226

Known bugs

• [Bug] Hands animation no longer working #234

Testing

In order to test this PR, do the following:

• Run the tests on your local machine with cargo test -p hotham --release -- --test-threads 1
• Run all the examples on a Quest 2

[ickk]

All the examples ran fine on my Quest 2 device, including the stress test!

[kanerogers]