Fixes for Retina & PixelRatio < 1

Incorporates & extends PR #461.

There were a few places where we assumed that `g.width_ == drawingContext.width`, but with `devicePixelRatio` that may not be true.

To test this I:

  1. Rendered a chart on both a retina and non-retina screen at 100% zoom. It looked crisp on retina and fine on non-retina.
  1. Zoomed out to 75% on non-retina. Issue #440 did not regress.