Add support for hidpi "retina" tile layers

[jnewmoyer]

Description

We are using tiled WMS basemaps that are published with 512px tile size and 900913x2 gridset in GeoServer.

In Leaflet, we can display these in "retina" mode using:

{ 'detectRetina': true, 'format_options': 'dpi:192' };

According to the OpenLayers docs this can also be done using WMTS or XYZ tiled layer using tilePixelRatio but I think this would need to be made available as a checkbox or config option in the layers.

There is also a hidpi setting in ImageWMS but that seems to be dependent on serverType and the Map's pixelRatio so not sure if that is a feasible route for this use case.

Acceptance criteria

• Layers can be configured to display using 2x "retina" tiles either through a checkbox in the UI or a config option (localConfig.js)