Texture atlas

* Combines `TextureAtlas` and `TextureAtlasBuilder`. 
* Makes `TextureAtlas` and `BillboardCollection.textureAtlas` private.
* Removes `Billboard.imageIndex` and adds `Billboard.imageUrl` and `Billboard.image`.

For #1688.

The functions in `TextureAtlas` were changed to mimic those that were in `TextureAtlasBuilder` though it may be better to return promises. See the use of the `TextureAtlas` in `LabelCollection` which expects the callbacks to be executed before returning.


Well, it depends.  If `b1` was loaded from a URL, then yes.  If `b1` was loaded from a canvas, its id is not a URL (and possibly a generate guid), and it will only work if a texture with that ID is in the atlas already.