setOrigin and setDestination changing others maps directions

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Hi,

I have 2 classes with 2 differents maps with differents containers.
When i made changes on one map, this is independant from the other map, excepts for setOrigin and setDestination. Those 2 functions impacts the two independants directions instances.

Class 1
```
this.map1 = new mapboxgl.Map({
container: 'map1',
style: 'mapbox://styles/mapbox/streets-v10',
});

this.directions1 = new MapboxDirections({
accessToken: mapboxgl.accessToken,
unit: 'metric',
profile: 'walking',
interactive: false,
controls: {
inputs: false,
instructions: false,
profileSwitcher: false
}
});
this.map1.addControl(this.directions);
...
this.directions1.setOrigin([lon, lat]);
this.directions1.setDestination([lon+1, lat+1]);
```

Class 2
```
this.map2 = new mapboxgl.Map({
container: 'map2',
style: 'mapbox://styles/mapbox/streets-v10',
});

this.directions2 = new MapboxDirections({
accessToken: mapboxgl.accessToken,
unit: 'metric',
profile: 'walking',
interactive: false,
controls: {
inputs: false,
instructions: false,
profileSwitcher: false
}
});
this.map2.addControl(this.directions);
...
this.directions2.setOrigin([lon+3, lat+3]);
this.directions2.setDestination([lon+3, lat+3]);
```

Problem : the setOrigin and setDestination from directions2 impacts the direction1 instance.

Reproduce the issue with the two MapboxDirections instances and the setOrigin and setDestination calls shown in the report. Inspect how those methods update direction state, then verify that changing directions2 leaves directions1 unchanged.

Written by the indexing model from the issue text.