Webtiles: scale dungeon canvas by devicePixelRatio.

On high DPI displays, this means that it can draw cells with the entire
available resolution, so that if the view needs to be scaled down on
those displays it can still turn up in its full detail.

If this is desirable, it can be extended to other displays.

Makes WebTiles usable properly on my Nexus 7 (2013). Hooray.