Proper scaling on HiDPI displays

Code based on:

https://github.com/mozilla/pdf.js/blob/c6c4583956b0b0c85aa6d5f8e20eacc16665f721/examples/text-selection/js/minimal.js#L51-L66

https://github.com/mozilla/pdf.js/blob/e294c8883a3da2f5012acaeb55be32b8fe57c53a/web/ui_utils.js#L93-L106

`context._scaleX`, `context._scaleY` is used internally by pdf.js

Fixes #27.

Can slow down rendering a bit, I'll try to make a patch for rendering only visible pages in next few days.