cast type of else statement - might work better with type guard

src/source/raster_dem_tile_worker_source.ts

@@ -1,14 +1,12 @@
 import DEMData from '../data/dem_data';
 import {RGBAImage} from '../util/image';
-import window from '../util/window';
 
 import type Actor from '../util/actor';
 import type {
     WorkerDEMTileParameters,
     WorkerDEMTileCallback,
     TileParameters
 } from './worker_source';
-const {ImageBitmap} = window;
 
 class RasterDEMTileWorkerSource {
     actor: Actor;
@@ -25,7 +23,7 @@ class RasterDEMTileWorkerSource {
     loadTile(params: WorkerDEMTileParameters, callback: WorkerDEMTileCallback) {
         const {uid, encoding, rawImageData} = params;
         // Main thread will transfer ImageBitmap if offscreen decode with OffscreenCanvas is supported, else it will transfer an already decoded image.
-        const imagePixels = (ImageBitmap && rawImageData instanceof ImageBitmap) ? this.getImageData(rawImageData) : rawImageData;
+        const imagePixels = (ImageBitmap && rawImageData instanceof ImageBitmap) ? this.getImageData(rawImageData) : rawImageData as RGBAImage;
         const dem = new DEMData(uid, imagePixels, encoding);
         this.loaded = this.loaded || {};
         this.loaded[uid] = dem;