Set default scale based on dpi

[mbrubeck]

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.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/1377

This is an external review system which you may optionally use for the code review of your pull request.

In order to help critic track your changes, please do not make in-place history rewrites (e.g. via git rebase -i or git commit --amend) when updating this pull request.

[metajack]

r+

[bors-servo]

saw approval from metajack
at mbrubeck@c036f17

[bors-servo]

merging mbrubeck/servo/hidpi = c036f17 into auto

[bors-servo]

mbrubeck/servo/hidpi = c036f17 merged ok, testing candidate = a7494f7

[bors-servo]

all tests pass:
success: http://servo-buildbot.pub.build.mozilla.org/builders/linux/builds/1273
success: http://servo-buildbot.pub.build.mozilla.org/builders/mac/builds/1227

[bors-servo]

fast-forwarding master to auto = a7494f7