Webgl points / get back the instructions array from the worker

This means we won't have to recreate a binary buffer (through a typed array)
on every `rebuildBuffer` call.

src/ol/worker/webgl.js

@@ -30,8 +30,9 @@ onmessage = event => {
       type: 'buffers-generated',
       vertexBuffer: vertexBuffer.buffer,
       indexBuffer: indexBuffer.buffer,
+      renderInstructions: renderInstructions.buffer,
       projectionTransform
-    }, [vertexBuffer.buffer, indexBuffer.buffer]);
+    }, [vertexBuffer.buffer, indexBuffer.buffer, renderInstructions.buffer]);
   }
 };