Remove waypoint

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Im trying to remove the waypoint after ive visited it, but im using custom waypoint markers using the waypointSymbolStyleLayer delegate method for navigationMapView.

```
var features = [MGLPointFeature]()
for waypoint in self.waypoints {
let feature = MGLPointFeature()
feature.coordinate = waypoint.coordinate
if let name = waypoint.name{
if name == "Pickup" || name == "Delivery"{
feature.attributes = ["type": name.lowercased()]
features.append(feature)
}
}
}
let deliveryImage = UIImage(named: "delivery")
let pickupImage = UIImage(named: "pickup")
let y = MGLShapeSource(identifier: "waypointLayer", features: features, options: nil)
mglSource = y
mapView.style?.addSource(mglSource)
mapView.style?.setImage(deliveryImage!.resizeImage(targetSize: CGSize(size: 40)), forName: "delivery")
mapView.style?.setImage(pickupImage!.resizeImage(targetSize: CGSize(size: 40)), forName: "pickup")
let x = MGLSymbolStyleLayer(identifier: "waypointLayer", source: mglSource)
x.iconImageName = NSExpression(forKeyPath: "type")
x.iconAllowsOverlap = NSExpression(forConstantValue: true)
x.iconIgnoresPlacement = NSExpression(forConstantValue: true)

return x
```

Thats my code inside the delegate. When I call .removeWaypoints, and then call .showWaypoints at the new legIndex, it crashes and tells me that the source "waypointLayer" is already there.

What am I doing wrong? Am I stylying my map wrong?

Start with the navigationMapView waypointSymbolStyleLayer delegate method and trace how removeWaypoints and showWaypoints handle the MGLShapeSource and MGLSymbolStyleLayer using the "waypointLayer" identifier. Reproduce the crash, then verify that updating or removing waypoints no longer attempts to add a source that already exists.

Written by the indexing model from the issue text.