Fix backend_svg.RendererSVG.draw_text to render urls

The SVG backend does nothing with Text object's URLs.  In particular, the rendering path below
backend_svg.RendererSVG.draw_text does nothing with the provided GraphicsContext's _url field.

This commit fixes that issue, causing Text objects' URLs to become &lt;a&gt; tags around the rendered text in the rendered SVG.