[cairo] The Qahirah Notebooks Project

[Lawrence D'Oliveiro]

I wrote:

> So here is the current collection of Qahirah notebooks:
> <https://github.com/ldo/qahirah_notebooks>. I have done one on cubic
> Bézier curves, and another on matrices.

I have added another one on text and fonts. This one illustrates the
meanings of the font-metrics and text-metrics fields (confused about
which way the ascent and y-bearing go?) and derives the formulas for
vertically centring text according to the different metrics.