Allow data tile source loader to return a value or a promise

examples/data-tiles.js

@@ -32,7 +32,7 @@ const map = new Map({
           context.strokeRect(0, 0, size, size);
           const data = context.getImageData(0, 0, size, size).data;
           // converting to Uint8Array for increased browser compatibility
-          return Promise.resolve(new Uint8Array(data.buffer));
+          return new Uint8Array(data.buffer);
         },
         // disable opacity transition to avoid overlapping labels during tile loading
         transition: 0,