List all families for a fontmap.
Note that the returned families are not in any particular order.
`PangoFontMap` also implemented the [[email protected]] interface for enumerating families.
| this |
a `PangoFontMap` |
| families |
location to store a pointer to an array of `PangoFontFamily` *. This array should be freed with g_free . |
| n_families |
location to store the number of elements in |