Display Mapbox layer above kepler layer

**Is your feature request related to a problem? Please describe.**
I'm currently displaying the direction of driving with an extra layer with the mapbox reference:
The problem is that the layer with the arrows is always below the kepler gl segments. As a workaround I reduced the opacity, but it would be nice to display it above the kepler layers.

**Describe the solution you'd like**
Make the layers accessible with the mapbox reference that it is possible to put the arrow layer in front of the other.

**Describe alternatives you've considered**
Make it possible to upload icons and use it on kepler gl layers.