Generate mesh in worker off the main thread

In general, the inputs to the mesh refinement algorithm are simple and easy to transfer onto a worker: shape, geotransform. And the resulting mesh, especially once backed by typed arrays (see https://github.com/developmentseed/deck.gl-raster/issues/23) is cheap to [transfer](https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Transferable_objects) between threads.