Update the overlay API so that required props can actually be required props

[vicapow]

Currently, the <MapGL> component clones it's children and adds the necessary props. This causes all props to be required as optional since they first get created once without the necessary props. To solve this, we can either:

1.) Use React's context feature.

<MapGL ...mapProps>
    <Overlay locations={locations}/>
</MapGL>

This would require changes to the implementation of existing overlays. It also means we wont be able to use PureRenderMixin since it doesn't know about context. It's also seems to be discouraged to use context in React in general.

2.) Pass the overlays to a callback properly of the <MapGL> component.

<MapGL ...mapProps overlays={(viewport) => {
  <Overlay ...viewport locations={locations} />
}/>

This would cause changes to the usage of overlays but requires no changes to the overlays themselves. It also has the advantage of supporting PureRenderMixin.

[vicapow]

see: f7741b0

[vicapow]

Ended up changing this up a bit from the original proposal. see: https://gist.github.com/vicapow/00017553e92f613d5361 for details

This has landed with: fe329bc