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