Allow the server to define what Mercator shaders to use for the terrain

Currently both the client and the server uses hardcoded settings for what Mercator Shaders to use for the terrain. It would be better if this was defined as attributes of the world entity, just like the actual terrain is defined in the "terrain" attribute.
As with all attributes, we need to be expressive without being too technical, so that the clients gets some leeway on how to represent this.
The surfaces are defined as layers, with layers with lower "id" being at the bottom (and in a 3d client rendered first). Apart from standard shaders such as the grass and the sand, we also support areas (such as the areas under the oaks and the path). The information sent from the server should therefore contain information both about how to do the "basic" terrain surfaces as well as how to do the areas (although in the latter case it would be sufficient to just specify "layer {id=7, type=area, material=path}" whereas in the grass layer definition we would also need to specify falloff similiar values, to be sent to the Mercator GrassShader).
I'm thinking of a map-type attribute on the world entity named "layers", the keys being the id of the layers themselves, and the corresponding value being another map with information about the layer.
Pseudocode:
layers:
    0:
        type: fill
        material: granite
    1:
        type: grass
        material: grass
        falloff: 7
 and so on