Proper scaling on HiDPI displays

Code based on:

https://github.com/mozilla/pdf.js/blob/c6c4583956b0b0c85aa6d5f8e20eacc16
665f721/examples/text-selection/js/minimal.js#L51-L66

https://github.com/mozilla/pdf.js/blob/e294c8883a3da2f5012acaeb55be32b8f
e57c53a/web/ui_utils.js#L93-L106

context._scaleX, context._scaleY is used internally by pdf.js
Merge pull request #39 from saidai-no/hidpi

Proper scaling on HiDPI displays