View scaling

Provides consistent scaling of tiles across screens of different pixel densities and orientations and reorganizes View class to clarify functionality. 
It is a raster tile convention, yes. It *is* however also relevant to the data, as the server simplifies geometries to remove any sub-pixel detail from the geometries, under the assumption of a 256px tile. Note that this means our tiles are currently over-simplified for displays with a device pixel ratio > 1 (i.e. "retina")... there are also some raster conventions for addressing that, but we have not yet chosen if we will adopt them.

We may want to experiment with non-256 sizes, but we need to be aware of the geometry simplification and selection implications.