Respect text-offset in line-placed symbol collision handling

@ChrisLoer is already working on this.

Since May 5, 2018.

We currently do not respect the value of `text-offset` when determining whether two symbol instances collide.

Example -- in this style:

```js
{
"version": 8,
"metadata": {
"test": {
"height": 64,
"collisionDebug": true
}
},
"center": [ 0, 0 ],
"zoom": 0,
"sources": {
"geojson": {
"type": "geojson",
"data": {
"type": "Feature",
"properties": {},
"geometry": {
"type": "LineString",
"coordinates": [
[-20, -20],
[20, -20],
[20, 20],
[-20, 20]
]
}
}
}
},
"glyphs": "mapbox://fonts/mapbox/{fontstack}/{range}.pbf",
"layers": [
{
"id": "guid",
"type": "line",
"source": "geojson"
},
{
"id": "text",
"type": "symbol",
"source": "geojson",
"layout": {
"symbol-placement": "line",
"symbol-spacing": 115,
"text-size": 10,
"text-allow-overlap": false,
"text-ignore-placement": false,
"text-field": "A",
"text-font": [
"Open Sans Semibold",
"Arial Unicode MS Bold"
],
"text-keep-upright": false,
"text-offset": [0, -3]
}
}
]
}
```

We'd expect only one of the 'A' instances to be rendered, but both are: