Emit a 'radiuschanging' event

[eddydg]

For a more advanced styling I'd need to keep track of a circle whose radius is being dragged.
Currently we only have 'radiuschanged' which is emitted once the drag has ended. So a 'radiuschanging' could come in handy.

To be consistent, a 'centerchanging' could also be added.

What do you think? would you accept a PR with this feature?

@eddydg Do you mean it should fire repeatedly every time we get a rAF slot during animation? Sounds like it would degrade performance (one of our biggest challenges).

[mblomdahl]

Please put up a pull request if you have a good strategy for adding this feature!

[eddydg]

Thank you for your answers. I'm not sure what a rAF slot is.
I was thinking about something very (too?) simple: adding

this._eventEmitter.emit('radiuschanging', this); in the set radius and
this._eventEmitter.emit('centerchanging', this); in the set center.

PR: #77

I am not seeing any visible difference in performance but I am keeping a very light playground: http://lab.meltedpeng.com/mapbox-gl-circle/

Edit: Oh ok, rAF for requestAnimationFrame. Never used this abbreviation.
It fires when the center changes, so reacting to the _onRadiusHandlesMouseDown and _onRadiusHandlesMouseMove listeners.