Edge cases don't render correctly

[blampe]

Examples:

• markdeep | goat
• markdeep | goat

Markdeep's source became available after I hacked together this project many years ago. Its diagramToSVG method is worth referencing.

[dmullis]

Firefox allows setting per-window background to a color different from the "theme" background. Viewing this way the Markdeep example page cited above [lineends], note the white-filled circle at right (https://casual-effects.com/markdeep/features.md.html#diagramexamples/lineends)

Conclusions:

1. Like Goat, Markdeep too fills opaquely the center of the seemingly "hollow" circle specified by 'o'.
2. Visually corrupt results are easily fallen into by code that attempts to guess the window background color.