Convert a quadratic bezier to a cubic one

From https://fontforge.org/docs/techref/bezier.html#converting-truetype-to-postscript:

Any quadratic spline can be expressed as a cubic (where the cubic term is zero). The end points of the cubic will be the same as the quadratic’s.

CP₀ = QP₀
CP₃ = QP₂

The two control points for the cubic are:

CP₁ = QP₀ + 2/3 *(QP₁-QP₀)
CP₂ = QP₂ + 2/3 *(QP₁-QP₂)

…There is a slight error introduced due to rounding, but it is unlikely to be noticeable.