[Feat] Multi view terrain extension

Target Use Case

Hi, so in my project I modified terrain extension in a simple way to support multi-view for terrain extension and since this was marked as TODO I tought I will sent you what I did to see if that would be possible to add in the deck.gl library.

Proposal

So in terrain-effect.ts in preRender I modified my function to look like this :

`
preRender(opts: PreRenderOptions): void {
// @ts-expect-error pickZ only defined in picking pass
if (opts.pickZ) {
// Do not update if picking attributes
this.isDrapingEnabled = false;
return;
}

const { viewports } = opts;
const isPicking = opts.pass.startsWith('picking');
this.isPicking = isPicking;
this.isDrapingEnabled = true;

// TODO - support multiple views?

viewports.forEach((viewport) => {
  const layers = (
    isPicking ? this.terrainPickingPass : this.terrainPass
  ).getRenderableLayers(viewport, opts as TerrainPickingPassRenderOptions);

  const filteredLayers = layers.filter((l) => l.id.startsWith(viewport.id));

  const terrainLayers = filteredLayers.filter(
    (l) =>
      l.props.operation.includes('terrain') && l.id.startsWith(viewport.id)
  );
  if (terrainLayers.length === 0) {
    return;
  }

  if (!isPicking) {
    const offsetLayers = layers.filter(
      (l) => l.state['terrainDrawMode'] === 'offset'
    );
    if (offsetLayers.length > 0) {
      this._updateHeightMap(terrainLayers, viewport, opts);
    }
  }

  const drapeLayers = filteredLayers.filter(
    (l) => l.state['terrainDrawMode'] === 'drape'
  );
  this._updateTerrainCovers(terrainLayers, drapeLayers, viewport, opts);
});

}
`

So the way it works in my app is I set a viewport id for my main and my second screen and for the layers I start their name by the viewport id I want them to show in, and then I use it in the layer filter to show what I want in the good screen. So this is based on the same logic that a layer that you want to see in a screen should be prefixed by that screen id.
Maybe this isn't the cleanest solution but it works quite well.

Start in terrain-effect.ts at preRender and trace how viewports, terrain passes, and renderable layers are handled. Compare the current single-view behavior with the proposed multi-view filtering approach. Done means terrain offset and drape layers render correctly across multiple views without breaking picking behavior; no test file is mentioned in the issue.