Implement missing CanvasRenderingContext2D methods (#44)
Implement missing CanvasRenderingContext2D methods (#44)

This change improves support for the CanvasRenderingContext2D API.

Most of the methods use floating-point parameters, since the API accepts
those. Pixel manipulation methods and ImageData are the exception; they
use integer parameters because they deal with discrete pixels.

Methods are better organized into categories, and sorted by name within
each category.

Reference: https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D.
dominikh/go-js-dom