Could we elevate fills above terrain using fill-z-offset (e.g. with fill-elevation-reference)?

Nobody has claimed this yet.

## Motivation

I want to be able to elevate any mapbox layer on a map with terrain using the new "z-offset" options. It works well for line-z-offset and symbol-z-offset.

Currently I cannot elevate circles at all (see https://github.com/mapbox/mapbox-gl-js/issues/13311) and fill-z-offset only works from sea level so is not compatible with terrain.

Is there a reason there is no "fill-elevation-reference" equivalent?

## Design Alternatives

There is a workaround to use fill-extrusions but this means using a different layer type.

## Design

Add a "fill-elevation-reference" equivalent with options for "ground", "sea" etc which allows "fill-z-offset" to elevate relative to terrain.

Start by reviewing the existing line-z-offset and symbol-z-offset behavior, then compare it with fill-z-offset on terrain. Define how a fill-elevation-reference option such as ground or sea should interact with fill-z-offset; the work is done when fills can be elevated relative to terrain without requiring fill-extrusions.

Written by the indexing model from the issue text.