Try different SDF granularities

Nobody has claimed this yet.

We can get better compression ratios by reducing the granularity of the SDFs -- the following commit rounds each value to the nearest multiple of 8:

https://github.com/mapbox/fontserver/commit/c213c12cf1ff45b5de9f17f8818352e344e33285

Visually it's hard to see much difference once rendered and here is the space saving for a 0-255 range:

```
master 43124 bytes
compress-more 23208 bytes
```

This is a nice boost we can get now roughly for free since it doesn't affect the format/integrating projects -- later we may try other encodings for the bitmap (PNG, etc.) but those will be format-breaking.
### Next actions
- [x] Try this and other settings (4 and 2? 16 is where I started to see artifacts...) with a variety of different fonts
- [ ] This could be variable/env based so if you want a very high quality SDF you can set your settings that way

Start by comparing the referenced compression commit with the current SDF generation path, then try granularities of 4, 8, and 2 across several fonts. Define an environment or variable-based quality setting without changing the format, and verify the resulting compression savings and visual artifacts.

Written by the indexing model from the issue text.