Dashed line dashes are clipped & offset at tile boundaries

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**mapbox-gl-js version**: 0.40.1

### Steps to Trigger Behavior

1. Draw line with `line-dasharray: [0.1, 1.8]` and `line-cap: round` that crosses tile boundary

```
mapboxgl.accessToken = 'pk.eyJ1IjoicXVpY2tseXdpbGxpYW0iLCJhIjoibmZ3UkZpayJ9.8gNggPy6H5dpzf4Sph4-sA';
var map = new mapboxgl.Map({
container: 'map',
style: 'mapbox://styles/mapbox/streets-v9',
center: [ -122.63480377210192, 45.49858719777322],
zoom: 20.505
});

map.showTileBoundaries = true;

map.on('load', function () {

map.addLayer({
"id": "route",
"type": "line",
"source": {
"type": "geojson",
"data": {
"type": "Feature",
"properties": {},
"geometry": {
"type": "LineString",
"coordinates": [[-122.63332018999998, 45.49907065], [-122.63635848, 45.498128739999984]]
}
}
},
"layout": {
"line-join": "round",
"line-cap": "round"
},
"paint": {
"line-color": "#191",
"line-width": {
'base': 1.5,
'stops': [
[14, 5],
[18, 20],
],
},
"line-dasharray": [0.1, 1.8]
}
});
});
```

jsbin: http://jsbin.com/kiqinefopa/edit?html,output

### Expected Behavior

Dashes are shown at regular spacing regardless of tile boundary locations; no artifacts are visible at tile boundaries

### Actual Behavior

Dash positions appear shift between tiles and are clipped at tile boundaries. The effect varies with zoom level.


Start with the supplied jsbin reproduction and the mapbox-gl-js line layer using line-dasharray, line-cap, and a line crossing a tile boundary. Trace the tile-boundary rendering path and verify behavior at multiple zoom levels; done means dash spacing remains regular and no clipping or offset artifacts appear at tile boundaries.

Written by the indexing model from the issue text.