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

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

I was basing my code on the example from [Learning WebGL](http://learningwebgl.com/blog/?p=1786) and didn't even notice.

But actually it doesn't matter because the framebuffer is not scaled down or scaled up.