Set default scale based on dpi
auto merge of #2224 : mbrubeck/servo/hidpi, r=metajack

On high-density (e.g. "Retina") displays, this will set the default scale to 1 px per screen coordinate, rather than the current 1px per hardware pixel. This means that content on high-density displays will be rendered at a higher resolution but about the same physical size, compared to lower-density displays.