Sweep gradient is not working in SVGs

[francozanardi]

Description

When a image uses sweep gradient and it's rendered as svg, the final result is just black.

Steps to reproduce

from pictex import *

canvas = Canvas().size(200, 200).background_color(SweepGradient(["red", "yellow", "red"]))

canvas.render().save("sweep.png")
canvas.render_as_svg().save("sweep.svg")

sweep.png:

sweep.svg: