Improve closest-side / farthest-side calculation in ellipse()

[oslego]

Currently naively assuming center in 50% 50%
Consolidate compute method into CSSUtils for use by circle()

Acceptance tests:

• ellipse(closest-side at 10% 70%)
• ellipse(closest-side farthest-side at 10% 70%)
• ellipse(farthest-side closest-side at 10% 70%)
• ellipse(closest-side closest-side at -100px -100px) // yields zero-size ellipse