Add custom runtime-optimized 3D model format

[slimsag]

Build:

• Custom Mach blender plugin, not just for exporting models but for now only to do that.
• A new custom runtime-optimized 3D model format for Mach.

glTF, Model3D, or other model formats are explicitly out of the picture.

glTF is terrible for performance and not designed to be a runtime-optimized format, its binary format includes JSON payloads 🤮

Other formats have various weird quirks for their intended purposes (which is totally fine) but long-term we will be better served by one tailored to our use cases.

Long term, we will have deeper blender integration, e.g. mach blender plugin that can directly export the optimized format, where our renderer understands blender materials to some approximation etc.

[Atomk]

Why no longer Model3D? Just curious, reading their rationale it looks like a good candidate, even though it doesn't seem to be a very popular format (on their website Mach is listed as one of the four projects using the format). I understand the reasons you wrote here for discarding GLTF, but I could not find information on the decision about Model3D.