Hooks SVG stuff into the viewer

Don't merge -- just for reference.

This is pilot of connecting SVG stuff into the generic viewer. We have to slowly refactor to make this pretty.

Generate preview to build the extension with current master. There are "#svg=true" flag and `extensions.uriloader@pdf.js.svgRendering` preference, to enable the SVG rendering.
What about doing something like PDFJS.displayBackend?