gtkairo: cairo medium ellipse drawing and line styles

[David Lichteblau]

Gtkairo Bug 22:

The cairo version of medium-draw-ellipse* needs a rewrite.

(The gdk medium can draw ellipses just fine, however.)

The current implementation fills an ellipse, but completely fails when drawing its border:

  ;; This one is tricky. Cairo doesn't know ellipses, it only knows
  ;; circles. But then it is fully capable to draw circles under affine
  ;; transformations only that the line style is transformed too. So
  ;; what we do: We setup an [additional] transformation to from our
  ;; ellipse to a circle and setup line style properly transformed. ---
  ;; This is not entirely correct in case of shearing or odd scaling
  ;; transformations.

Unless cairo starts supporting ellipses natively, I would suggest using bezier curves to approximate an ellipse.