[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`~~
I was afraid that this would be a bit confusing. But you might have to create more than one new atlas: For example your current atlas is 128x128 and now you want to add a 500x500px image. You will have to create the 256x256 and the 512x512 atlas. That's why it is inside the loop.