Based on mapbox.normalizeTileURL() in mapbox/mapbox-gl-js, but insert a “{ratio}” placeholder instead of “@2x” in order to keep the pixelRatio logic in TileData::request().
Fixes#919.
we are now scoping all file requests to an environment object. The FileSource implementation treats
this as an opaque pointer, but allows canceling all Requests that are associated with that pointer.
This is necessary to abort all file requests that originated from a particular Map object. Aborting
a file request is different from canceling a file request: A canceled request doesn't have its
callback called, while an aborted request will have its callback called with an error, indicating
that the environment is going to be shut down.
