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.
Merge pull request #2521 from mdengler/svg-text-urls

Fix backend_svg.RendererSVG.draw_text to render urls