Webgl worker / add tests, some typing and documentation

The worker currently works by receiving GENERATE_BUFFERS messages and
will send back the same kind of message, with the generated buffers
attached. All properties of the original message are kept, so that
when a GENERATE_BUFFERS message comes back to the main thread it
is possible to know what and how the buffers where generated.

This is typically used for the `projectionTransform` matrix, and
will also be necessary when working with tiles.

test/spec/ol/worker/webgl.test.js

@@ -0,0 +1,46 @@
+import {create} from '../../../../src/ol/worker/webgl.js';
+import {WebGLWorkerMessageType, writePointFeatureInstructions} from '../../../../src/ol/renderer/webgl/Layer';
+
+
+describe('ol/worker/webgl', function() {
+
+  let worker;
+  beforeEach(function() {
+    worker = create();
+  });
+
+  afterEach(function() {
+    if (worker) {
+      worker.terminate();
+    }
+    worker = null;
+  });
+
+  describe('messaging', function() {
+    it('responds to GENERATE_BUFFERS message type', function(done) {
+      worker.addEventListener('error', done);
+
+      worker.addEventListener('message', function(event) {
+        expect(event.data.type).to.eql(WebGLWorkerMessageType.GENERATE_BUFFERS);
+        expect(event.data.renderInstructions.byteLength).to.greaterThan(0);
+        expect(event.data.indexBuffer.byteLength).to.greaterThan(0);
+        expect(event.data.vertexBuffer.byteLength).to.greaterThan(0);
+        expect(event.data.testInt).to.be(101);
+        expect(event.data.testString).to.be('abcd');
+        done();
+      });
+
+      const instructions = new Float32Array(100);
+
+      const message = {
+        type: WebGLWorkerMessageType.GENERATE_BUFFERS,
+        renderInstructions: instructions,
+        testInt: 101,
+        testString: 'abcd'
+      };
+
+      worker.postMessage(message);
+    });
+  });
+
+});