Set default stroke width for SVG shapes without it

[Eymeric65]

Following SVG standard, when a stroke color is supplied without a line stroke, a line stroke is applied by default. This copy the behavior to manim

Overview: What does this pull request change?

Motivation and Explanation: Why and how do your changes improve the library?

Basically I got some SVG that imported without line stroke and I got interested into how SVG worked, everything is parsed correctly just when you view it as an end user the browser apply a stroke on element that have stroke color even if they don't specify stroke width. Manim imported this without applying this underlying rule of SVG.

Any comment is welcome if I had wrongly interpreted SVG rule.

Links to added or changed documentation pages

Further Information and Comments

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested