Refactor /services loadTiles

[thomas-hervey]

Several services, including the image viewers like openstreetcam, mapillary, and streetside (along with notes and osm) require loading tiles. Besides checking for nullIsland (as the image viewer ones do), all of these files load tiles in a very similar way. See openstreetcam for example.

This could probably be refactored to a utility.

[bhousel]

Yes! Strong 👍 on this one..
It's been something I've wanted to do for a while.

We do kind of have a utility already in d3.geo.tile.js, so it might make sense to:

1. move a lot of the common code into there
2. while also adding the capability to use variable tile sizes (it's currently hardcoded to 256px)

For historical context, d3.geo.tile is an older library that we pulled into iD and changed a little bit. The version we use works fine for our use. An old version of it lives here and hasn't been updated in years. That d3 plugins repo is not maintained anymore.

I think the best way forward might be to move our internal lib/d3.geo.tile.js module into a /util/utilTile.js module with a new name so it wouldn't be confused with the old D3 v3 plugin, then give it the superpowers listed above.

[thomas-hervey]

@bhousel I once I have a stable working notes implementation, I can try to tackle this.

[thomas-hervey]

Several changes made in master after this merge including e4d829e d1fe81b ff64455 02713e4

[bhousel]

I'm still working on a few more cleanups, and supporting sizes other than 256px.