[webgl-point] Make WebGL image replay support icon sprites

This PR makes the WebGL image replay support icon sprites. It also adds an `icon-sprite-webgl` example allowing to test this.

The WebGL replay code is still quite ugly and prototypish.

What's missing:

* Support for image anchors
* Context loss handling
* Destroy of textures
* Texture upload management
* Clean separation/API between renderer and replay
* Layer opacity, hue, … in the fragment shader
* Precision/Jitter issue
* Minimize the amount of data sent to the GPU (see [comment](https://github.com/camptocamp/ol3/commit/bbea205a9ce8f76f4b415f6ca19d5f8e05448398#commitcomment-8384931) from @tsauerwein)