Webgl points / shifts the buffer write logic in a worker

The worker receives a transferable array of instructions
and sends back two transferable arrays (vertex and index buffer).
The projection transform is also sent so that when the main thread
receives the buffers from the worker it also knows which projection to
apply when rendering the geometries.

src/ol/worker/webgl.js

@@ -0,0 +1,38 @@
+/**
+ * @module ol/worker/webgl
+ * A worker that does cpu-heavy tasks related to webgl rendering
+ */
+import {POINT_INSTRUCTIONS_COUNT, POINT_VERTEX_STRIDE, writePointFeatureToBuffers} from '../renderer/webgl/Layer.js';
+
+onmessage = event => {
+  if (event.data.type === 'generate-buffer') {
+    const renderInstructions = new Float32Array(event.data.renderInstructions);
+    const customAttributesCount = event.data.customAttributesCount || 0;
+    const instructionsCount = POINT_INSTRUCTIONS_COUNT + customAttributesCount;
+    const projectionTransform = event.data.projectionTransform;
+
+    const elementsCount = renderInstructions.length / instructionsCount;
+    const indexBuffer = new Uint32Array(elementsCount * 6);
+    const vertexBuffer = new Float32Array(elementsCount * 4 * (POINT_VERTEX_STRIDE + customAttributesCount));
+
+    let bufferPositions = null;
+    for (let i = 0; i < renderInstructions.length; i += instructionsCount) {
+      bufferPositions = writePointFeatureToBuffers(
+        renderInstructions,
+        i,
+        vertexBuffer,
+        indexBuffer,
+        bufferPositions,
+        instructionsCount);
+    }
+
+    postMessage({
+      type: 'buffers-generated',
+      vertexBuffer: vertexBuffer.buffer,
+      indexBuffer: indexBuffer.buffer,
+      projectionTransform
+    }, [vertexBuffer.buffer, indexBuffer.buffer]);
+  }
+};
+
+export let create;