Webtiles: scale canvases by devicePixelRatio properly.

This is a new version of 75d9865; the block in dungeon_renderer.shift is
the fix for the issue that was preventing this from working properly.

As a bonus, it scales more canvases, too!

It doesn't scale the minimap yet, though. Not that blocks need scaling.
Webtiles: Modify styling to prevent canvas blurriness on HiDPI screens where possible.
Add zuurr to mailmap and CREDITS.txt.