Add an optional layer "land" (inverted "water")

[indus]

In a mapclient I need to sandwich a custom layer of ocean currents in between "water" and "land" in such a way that "land" masks the custom layer. Therefor it has to be ontop of the of the "water" background. But tilesets typically provide "water" on top of a "land" background.

It would be really nice if you could add this feature to the schema (propably optional) or give me a hint on how to process such a "land" layer.

There is a related issue that tries to solve this in the client: https://github.com/mapbox/mapbox-gl-js/issues/993