[feature request] support mapgl as a rendering platform

[tim-salabim]

See walkerke/mapgl#57 for some previous discussion

[tim-salabim]

In f396ee2 I've

• set up a new mapviewOption(platform = "mapgl")
• added a function skeleton for sf objects (let's start with those, we can add sfc and others later)
• added a dev script for testing and dev purposes.