The uuid to replace a family FROM. canvas's substitute_font() takes the source as a uuid and
matches every reference by that uuid OR by the family it resolves to, so any uuid that resolves to
the family will do: the one its references carry, else one of them, else the uuid the FontManager
has for the family. Null when there is none, and the row cannot be replaced.
The uuid to replace a family FROM. canvas's substitute_font() takes the source as a uuid and matches every reference by that uuid OR by the family it resolves to, so any uuid that resolves to the family will do: the one its references carry, else one of them, else the uuid the FontManager has for the family. Null when there is none, and the row cannot be replaced.