Refactor: using a new scaling approach.

No longer switching immediately on the FPS reaching below/above
certain thresholds. Instead, FPS decay has been disabled, and
high/low framerates will lerp the "goalScale" closer to the next
appropriate available one. Then, the ideal scale is simply:

```
scale = Math.round(goalScale)
```

This appears to be less randomly sensitive to changes in framerate,
and recovers much more quickly compared to the previous approach.
slight tweak to test/index.js
2.0.0