[cairo] Ellipse stroke

[Richard Copley]

> On the top right and bottom right: avoiding the bug by stroking the
> arc from t to pi-t, where cos²(t) = (r₀² - ∛(r₀r₀L)²) / (r₀² - r₁²),
> where r₀ and r₁ are the ellipse semiaxes and L is the line width.
>
> (That equation gives the locations of the cusps (if any) in the inner
> curve of the ellipse stroke (proof omitted).)

Typo, sorry. The equation should be:

cos²(t) = (r₀² - ∛(r₀r₁L)²) / (r₀² - r₁²)