Fix typings in workers

The worker self object is simply cast to any to make sure typescript
does not complain.
Actual typecheck may be restored once the following issue is fixed:
https://github.com/microsoft/TypeScript/issues/20595

src/ol/worker/version.js

@@ -4,9 +4,12 @@
  */
 import {VERSION} from '../util.js';
 
-onmessage = event => {
+/** @type {any} */
+const worker = self;
+
+worker.onmessage = event => {
   console.log('version worker received message:', event.data); // eslint-disable-line
-  postMessage(`version: ${VERSION}`);
+  worker.postMessage(`version: ${VERSION}`);
 };
 
 export let create;

src/ol/worker/webgl.js

@@ -10,7 +10,10 @@ import {
 } from '../renderer/webgl/Layer.js';
 import {assign} from '../obj.js';
 
-onmessage = event => {
+/** @type {any} */
+const worker = self;
+
+worker.onmessage = event => {
   const received = event.data;
   if (received.type === WebGLWorkerMessageType.GENERATE_BUFFERS) {
     const renderInstructions = new Float32Array(received.renderInstructions);
@@ -41,7 +44,7 @@ onmessage = event => {
       renderInstructions: renderInstructions.buffer
     }, received);
 
-    postMessage(message, [vertexBuffer.buffer, indexBuffer.buffer, renderInstructions.buffer]);
+    worker.postMessage(message, [vertexBuffer.buffer, indexBuffer.buffer, renderInstructions.buffer]);
   }
 };