Render texture in a 3D space

[define-private-public]

Discussed this in #33, required for #13

Now that we have Entity3D, we can easily define a way to render a texture in a 3D space. But we need that proc now...

I'll go and work on it this week.

TODO list:

• [ ] Proc needs to take in straight up vectors
• [ ] Write a test/example
• [ ] Would this make the 2D one obsolete then?