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.

src/ol/renderer/webgl/Layer.js

@@ -5,6 +5,26 @@ import LayerRenderer from '../Layer.js';
 import WebGLHelper from '../../webgl/Helper.js';
 
 
+/**
+ * @enum {string}
+ */
+export const WebGLWorkerMessageType = {
+  GENERATE_BUFFERS: 'GENERATE_BUFFERS'
+};
+
+/**
+ * @typedef {Object} WebGLWorkerGenerateBuffersMessage
+ * This message will trigger the generation of a vertex and an index buffer based on the given render instructions.
+ * When the buffers are generated, the worked will send a message of the same type to the main thread, with
+ * the generated buffers in it.
+ * Note that any addition properties present in the message *will* be sent back to the main thread.
+ * @property {WebGLWorkerMessageType} type Message type
+ * @property {ArrayBuffer} renderInstructions Render instructions raw binary buffer.
+ * @property {ArrayBuffer=} vertexBuffer Vertices array raw binary buffer (sent by the worker).
+ * @property {ArrayBuffer=} indexBuffer Indices array raw binary buffer (sent by the worker).
+ * @property {number=} customAttributesCount Amount of custom attributes count in the render instructions.
+ */
+
 /**
  * @typedef {Object} PostProcessesOptions
  * @property {number} [scaleRatio] Scale ratio; if < 1, the post process will render to a texture smaller than

src/ol/renderer/webgl/PointsLayer.js

@@ -7,7 +7,7 @@ import {DefaultAttrib, DefaultUniform} from '../../webgl/Helper.js';
 import GeometryType from '../../geom/GeometryType.js';
 import WebGLLayerRenderer, {
   getBlankTexture,
-  POINT_INSTRUCTIONS_COUNT, POINT_VERTEX_STRIDE,
+  POINT_INSTRUCTIONS_COUNT, POINT_VERTEX_STRIDE, WebGLWorkerMessageType,
   writePointFeatureInstructions
 } from './Layer.js';
 import ViewHint from '../../ViewHint.js';
@@ -276,10 +276,11 @@ class WebGLPointsLayerRenderer extends WebGLLayerRenderer {
 
     this.worker_ = createWebGLWorker();
     this.worker_.addEventListener('message', function(event) {
-      if (event.data.type === 'buffers-generated') {
-        const projectionTransform = event.data.projectionTransform;
-        this.verticesBuffer_.fromArrayBuffer(event.data.vertexBuffer);
-        this.indicesBuffer_.fromArrayBuffer(event.data.indexBuffer);
+      const received = event.data;
+      if (received.type === WebGLWorkerMessageType.GENERATE_BUFFERS) {
+        const projectionTransform = received.projectionTransform;
+        this.verticesBuffer_.fromArrayBuffer(received.vertexBuffer);
+        this.indicesBuffer_.fromArrayBuffer(received.indexBuffer);
         this.helper_.flushBufferData(this.verticesBuffer_);
         this.helper_.flushBufferData(this.indicesBuffer_);
 
@@ -415,11 +416,15 @@ class WebGLPointsLayerRenderer extends WebGLLayerRenderer {
       );
     }
 
-    this.worker_.postMessage({
-      type: 'generate-buffer',
-      renderInstructions: this.renderInstructions_.buffer,
-      projectionTransform: projectionTransform
-    }, [this.renderInstructions_.buffer]);
+    /** @type import('./Layer').WebGLWorkerGenerateBuffersMessage */
+    const message = {
+      type: WebGLWorkerMessageType.GENERATE_BUFFERS,
+      renderInstructions: this.renderInstructions_.buffer
+    };
+    // additional properties will be sent back as-is by the worker
+    message['projectionTransform'] = projectionTransform;
+
+    this.worker_.postMessage(message, [this.renderInstructions_.buffer]);
   }