draw_line_string from: is not usable due to float precision

Nobody has claimed this yet.

**mapbox-gl-js version**: 0.45.0
**mapbox-gl-draw version**: master

### Steps to Trigger Behavior
1. Click on a vertex in a customised `direct_select` mode
2. Call changeMode to continue the line, using `draw_line_string`'s `from` option
3. It won't work, due to float precision.

### Expected Behavior
Continue line should work.

### Actual Behavior
Continue line should not work.

OK, let me explain this in a bit. I'd like to continue a line using metaClick on a vertex at one of the endpoints endpoint in a customised direct_select.

```
onVertex = function(state, e) {
const about = e.featureTarget.properties

if (e.originalEvent.metaKey && state.feature.type === 'LineString') {
const clickedIndex = Number(about.coord_path)
if ([0, state.feature.coordinates.length - 1].includes(clickedIndex)) {
return this.changeMode('draw_line_string', { featureId: state.feature.id, from: e.featureTarget })
}
}
```

There are 2 issues with this.

1. The code is actually wrong, this line https://github.com/mapbox/mapbox-gl-draw/blob/bf3dac14a7439f9c99e179c37f1f59a218b3a319/src/modes/draw_line_string.js#L22 should be

```
from = from.geometry.coordinates
```

2. The vertices are compared by geometric location, not by index. Comparing by geometric location doesn't work for some reason, I don't know if this is a bug in mapbox-gl core or in mapbox-gl-draw, but coordinates only match up till 6 digits.

Anything over 6 digits shouldn't be used in GeoJSON (as per recommendation of the specs), and the [7th digit is for 11 mm](https://gis.stackexchange.com/questions/8650/measuring-accuracy-of-latitude-and-longitude/8674#8674), but still, inside memory these coordinates should match I believe.

3. I actually liked the proposed API much better, where you submitted an index to `from:`, or at least that's what I thought was proposed here: https://github.com/mapbox/mapbox-gl-draw/issues/605#issuecomment-284491132

Why was that nice, logical proposal of using index changed into this passing of a full geometric feature and then comparing using location?

BTW, based on 1. and 2. I believe probably no one is using this feature and can be changed to use index without effecting users. Alternatively "fromIndex" could be introduced and "from" can be deprecated I guess.

Start in src/modes/draw_line_string.js at the linked line and trace how the draw_line_string from option compares vertices. Reproduce continuation from a customised direct_select mode, then determine whether the existing from behavior or a separate fromIndex API should be retained. Done means continuing a line works without the reported float-precision failure and the chosen API behavior is documented by the implementation.

Written by the indexing model from the issue text.