Add Hashable interface
Implement hashCode for ol.style.Circle
Add atlas manager
Double the size when creating new atlases
Add constant `ol.has.WEBGL_MAX_TEXTURE_SIZE`
Replace hashCode with checksum

Hash codes are not collision free, so what we
actually need is a checksum.
Only create new atlas after testing all existing
Use id string instead of hash code
Take `space` into account when checking size
Move AtlasManager to `ol.style.AtlasManager`
Add AtlasManagerOptions and improve docs
Merge pull request #14 from tsauerwein/webgl-point-atlas-manager

[webgl-point] Add atlas manager