Base class for font faces.

A [[cairo::font_face_t]] represents a particular font at a particular weight, slant,
and other characteristic but no size, transformation, or size.

Font faces are created using font-backend-specific constructors, typically of
the form cairo::backend_font_face::create(), or implicitly using the toy text
API by way of [[cairo::select_font_face]]. The resulting face can be accessed
using [[cairo::get_font_face]].

