cairo-set-scaled-font (cr font)

cr -- a cairo-t context
font -- a cairo-scaled-font-t instance

Replaces the current font face, font matrix, and font options in the Cairo context with those of the cairo-scaled-font-t instance. Except for some translation, the current CTM of the Cairo context should be the same as that of the cairo-scaled-font-t instance, which can be accessed using the cairo-scaled-font-get-ctm function.