Make Vulkan backend working

[codeonwort]

Checklist

• [x] Fix crash for Vulkan null renderer
• [x] Run DearImgui on Vulkan
• [ ] Fill missing Vulkan implementation (search for comments with #todo-vulkan)
  • In between, refactor common API to gracefully map to both DX12 and Vulkan
• [ ] Replace static linking with dynamic linking for Vulkan library (hopefully DX12 also in future of future)

Never ending story

1. Get bored with filling Vulkan code.
2. Add more DX12-like code to the common interface.
3. More Vulkan TODOs are generated.
4. A few months later: let's work on Vulkan, oh but I forgot how Vulkan API works.

References

• https://github.com/microsoft/DirectXShaderCompiler/blob/main/docs/SPIR-V.rst

[codeonwort]

Almost 1.5 years since creating this issue, today it finally manages to render dear imgui with Vulkan 🫠