[WIP] 3D Textures

Implementation of 3D textures using PyOpenGL.
The example might look cooler if you modulate the commented random `I` above with this, like:
```
I = np.random.uniform(0, 1, (W, H, D)).astype(np.float32)
I *= np.linspace(0., 1., S)[np.newaxis, np.newaxis, :]
```
WDYT?