This implements the gradients for:
- CanvasRenderingContext2D.createLinearGradient()
- CanvasRenderingContext2D.createConicGradient()
- CanvasRenderingContext2D.createRadialGradient()
As loosely defined in: https://html.spec.whatwg.org/multipage/canvas.html#fill-and-stroke-styles
(It's really not very well defined for radial gradients)
Actual implementation (for radial gradients) was done with a lot
of trial and error, then visually comparing to other browsers.
