dither given canvas element in place

[xosofox]

For a project I needed to dither an already existing canvas, so I added this entry point for window.HTMLCanvasElement

Also added a canvas with a simple gradient to the client demo

[coveralls]

Pull Request Test Coverage Report for Build 18

• 14 of 22 (63.64%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.8%) to 94.581%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/client.js	5	13	38.46%
<!--	Total:	14	22	63.64%	-->

Totals
Change from base Build 14:	-1.8%
Covered Lines:	384
Relevant Lines:	406

---

💛 - Coveralls