Adds a new style custom layer.
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.
A string describing an error if the operation was not successful, or empty otherwise.
The custom layer host.
custom layer host
A style layer identifier.
If not empty, the new layer will be positioned according to layer position parameters.