WebWorker canvas renderer

[IvanSanchez]

Apparently it's possible to have canvas renderers inside a web worker thread, see https://stackoverflow.com/questions/1864756/web-workers-and-canvas

It would be nice to see if this could be applied to Leaflet's canvas renderers, as it would increase rendering speed of complex vector tiles here.

[IvanSanchez]

Also: https://github.com/casperlamboo/canvas-webworker

[desean1625]

Even better than that with the new chrome use offscreen canvas object.

https://developers.google.com/web/updates/2018/08/offscreen-canvas

[liyuanqiu]

+1, really need it for heavy render tasks