[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`~~
We could clear the space in the canvas and and an entry in the `emptyBlocks` array.

But I think this case should not occur in the way we will use the atlas manager. If the hashes (or better: the checksums) of two styles are equal, then also the styles are the same. So you have no interest in adding them twice to the atlas.