viewer.color_by is slow for large molecules

This actually looks cool, although it's probably not what we want :)

**To reproduce:**
In a notebook, execute this cell:
```python
mol = mdt.from_pdb('3aid')
drug = mol.chains['A'].get_ligand()
viewer = mol.draw(display=True)
viewer.stick()
```

Then this one:
```python
viewer.color_by(lambda atom:atom.distance(drug))
```

In the latest version, the colors draw in 1-at-a-time, which takes a while:

In 0.7.3, the color comes in all-at-once after thinking for a second or two: