This directory is a bit special in that we will mix gtk2 and gtk3 related
code - as long as it has to do with cairo.

Ideally we could offer variants for both gtk2 and gtk3, and perhaps gtk4
as well, but I don't fully know all changes that happened in cairo, so
someone else may have to provide the code for this. Personally I am fine
that we don't have feature parity in regards to gtk2 and gtk3.
