Road name label density badly calculated based on text-size

@ChrisLoer is already working on this.

Since Apr 6, 2018.

v0.44.1 @ Chrome 65 @ Windows 10:

### Steps to Trigger Behavior

1. I have created custom map data and style in Casablanca, and as road name labels in Mapbox examples look bad in that area it can most likely be reproduced with your data also.
2. I have excluded mostly every other layer for this to be more obvious.
3. I keep everything as a constant, only thing that changes is text-size : stops for label layer.

### Expected Behavior
with:
```
"text-size": {
"stops": [
[ 12, 10 ],
[ 19, 10 ],
[ 20, 16 ]
]
}
```
### Actual Behavior
Now i only change this:

```
"text-size": {
"stops": [
[ 12, 10 ],
[ 14, 10 ],
[ 20, 16 ]
]
}
```

And result is this:


How is this possible on zoom level 12, especially with stop [ 14, 10 ]?
And even better how does stop [ 19, 10 ], fixes this problem?
This is simplified example but this effect destroys map quality in certain areas of the globe where there are less POIs and other labels.