Port the NanoVG demo to WebGL

[pcwalton]

It'd be neat if the NanoVG demo could run in the browser, to show how to use Pathfinder as a canvas renderer on the Web.

[s3bk]

After looking at it, the main problem will be fonts. My WebGL demos all use a pure rust font parser and avoids the canvas API.

One approach could be to use a different CanvasFontContext on WebGL.

• CanvasFontContext::from_system_source() will definitly not be avaible.