[Feat]add accessor `getElevationBase` for `MVTLayer`

[lianzhao]

Target Use Case

mapbox-gl provide fill-extrusion-base and fill-extrusion-height to draw extruded polygon (like buildings).

The MVTLayer does not have getElevationBase accessor like mapbox-gl's fill-extrusion-base. I know that deck.gl support 3D polygons, but Mapbox Vector Tile spec does not support altitude yet.

Proposal

const mvtLayer = new MVTLayer({
    extruded: true,
    getElevation: f => f.properties?.["fill-extrusion-height"] || 0,
    getElevationBase: f => f.properties?.["fill-extrusion-base"] || 0,
})