[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`~~
> Additionally, you should take into account this.space_ when checking image size.

Good catch, thanks!

About your suggestion: I guess there are better strategies. But just as a reminder: This atlas manager is supposed to deal with icons, which usually have a size between 16 and 32 pixels. The case we are discussing is about having an "icon" of >1024px.