Restore original canvas context when destroying

[nnnick]

Fixes #713

Inspired by the work done in #847 but addresses the use case of resetting CSS width/height regardless of whether we have a devicePixelRatio or not.

By setting the width/height of the canvas itself, the spec says it should completely reset any of the state applied to the canvas context.

Section 4.8.11:

When the canvas element is created, and subsequently whenever the width and height attributes are set (whether to a new value or to the previous value), the bitmap and any associated contexts must be cleared back to their initial state and reinitialized with the newly specified coordinate space dimensions.

[etimberg]

+1 I will close #847