Adds a new [style custom layer](https://docs.mapbox.com/mapbox-gl-js/style-spec/#layers). Note: This is an experimental API and is a subject to change. Whenever a new style is being parsed and currently used style has persistent layers, an engine will try to do following: - keep the persistent layer at its relative position - keep the source used by a persistent layer - keep images added through `addStyleImage` method In cases when a new style has the same layer, source or image resource, style's resources would be used instead and `MapLoadingError` event will be emitted.
Return
A string describing an error if the operation was not successful, or empty otherwise.
Parameters
layerHost
The `custom layer host`.
layerId
A style layer identifier.
layerPosition
If not empty, the new layer will be positioned according to `layer position` parameters.