A closed path of four cubics that is the four-cubic circle under an
affine map (an ellipse, or a circle scaled out of proportion) is written
as a native ELLIPSE in R13 and later, with its center, major axis and
minor to major ratio. The check maps the path back onto the unit circle
and runs the existing circle recognition on it. R12 keeps the existing
fallback, and ellipses with a ratio under 1e-6 stay splines. The ratio
and end parameter are written in full so that a thin ellipse keeps its
size and a whole turn reads back closed.
Part of #1299.
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