RepeatingBitmapPaintStyle tiles a bitmap passed on passed in parameters
and paints according to that.
OffsetPaintStyle offsets an existing paint style and paints with that.
These two will be useful in the PDF renderer.
This also slightly refactors things to share more implementation with
the SVG linear gradients, and improve accuracy (which fixes some banding
issues).
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.
