Comment 2 for bug 1009765

So I did some more digging:

1. Apparently a quadratic can't be substituted by a cubic where both cubic controls are set equal to the single quadratic one, or one cubic control is set equal to a node and the other is set equal to the quadratic control. Only the 2/3 formula will work. See attached images. (http://processingjs.nihongoresources.com/bezierinfo/)

2. FWIW I submitted some simple patches for Cairo to support quadratics: http://lists.cairographics.org/archives/cairo/2012-August/023397.html