[webgl-point] Add atlas manager

 * Introduces a `checksum` 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`~~
Very minor, but we usually define `i` and ``i`` outside the loop, although not everyone does it maybe. I do it because my editor's linter complains. This is not enforced by our integration/build tools though, so I wouldn't complain if you leave it like this :-)