Switch off camera height change with terrain

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## Motivation

I am writing geospatial app with deck.gl and react-map-gl. The problem when in 3D mode ( mapbox terrain feature ) is that mapbox changes camera height depending on terrain height. Because deck.gl doesn't know about this camera height change, objects rendered by deck.gl are not aligned with terrain despite having correct z coordinate (height). This is even more obvious while camera is moving ( rotating, panning, changing pitch) it look like deck.gl object are moving in space.
Is there a way to switch off this camera behaviour.

## Design

I would like to to be able to configure mapbox-gl camera not to change height with terrain bellow it.

### Mock-Up

### Concepts

### Implementation

The issue names no source files, entry points, or tests. Start by tracing terrain-related camera handling in Mapbox GL JS and the interaction described with deck.gl and react-map-gl; done means providing a documented configuration that prevents terrain from changing camera height while preserving expected camera behavior.

Written by the indexing model from the issue text.