Use "none" instead of "transparent" when painting in SVG

Use "none" instead of "transparent" when painting in SVG

The SVG 1.1 spec, which SVGs generated by gvrender_core_svg.c are
labeled as, does not have the color "transparent" available which is
causing issues with SVG 1.1 parsers.

When specifying paint in the fill or stroke attributes, use "none"
instead of "transparent".  When specifying a gradient color that has
zero opacity, use that color at zero opacity instead of "transparent".
When specifying a gradient with the graphviz transparent color, use
a zero-opacity black as this is how SVG interprets "transparent" in
gradients in SVG 2.0.