[webgl-point] WIP: Add hit-detection support for WebGL

TODO:

 - [x] Remove `Style.getHitDetectionOffset` and adapt the `AtlasManager`
 - [x] Use hit-detection textures
 - [ ] Add a proper example


The callback provided by the developer may choose to stop hit detection
by returning a truthy value. This mechanism is not obvious.

Currently, it looks like the hit detection stops on first feature found.