[webgl-point] Add atlas manager

 * Introduces a `hashCode` function for `ol.style.Circle`
 * Adds an atlas manager that uses the packing algorithm described in  http://www.blackpawn.com/texts/lightmaps/

Currently still missing:
 * The hash value is not cached.
 * ~~When the atlas manager creates a new atlas, it uses the same size (default: 256x256px). We could double the size every time we create a new atlas.~~
 * ~~Add `ol.WEBGL_MAX_TEXTURE_SIZE` to `ol.has`~~
[Wikipedia](http://en.wikipedia.org/wiki/Texture_atlas) says *a texture atlas is a large image containing a collection, or "atlas", of sub-images*. In our case the AtlasManager is a collection of texture atlases. So the name *Atlas* would hide the fact that it is actually a collection of atlases.

And if we hide this from the user it will be difficult to communicate the meaning of `size` and `maxSize`.