Manual model loading

[ashpieboop]

For when you want to use an obj model for other renders than blocks or item.

[comp500]

This PR doesn't seem to stitch model textures properly. Texture stitching happens at the end of the construction of ModelLoader, before ModelLoaderMixin::init runs, so model textures aren't part of the texture atlas.

[ashpieboop]

@comp500 thanks for catching that! I'm going to add a use case in the test mod to highlight this, then I'll fix it.