[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


The next time `Map.renderFrame` is called, the framebuffer is set back to `null`:
https://github.com/camptocamp/ol3/blob/webgl-point/src/ol/renderer/webgl/webglmaprenderer.js#L506