Store Meshes purely on GPU

[codebycruz]

Currently, there's a draw call for each entity. Instead, we could upload all meshes to the gpu and refer to them by index, like how the current TextureManager works.

This requires #17

Would increase performance a ton.

[codebycruz]

Global Meshes, and then each Asset should use "Primitives" / "Sub Meshes" which are parts of the mesh with a different material. So this should supercede the current material groups implementation that I came up with, when this started out with obj files.