Sets the font map to use for Pango rendering.
The font map is the object that is used to look up fonts. Setting a custom font map can be useful in special situations, e.g. when you need to add application-specific fonts to the set of available fonts.
When not set, the widget will inherit the font map from its parent.
| this |
a `GtkWidget` |
| font_map |
a `PangoFontMap`, or null to unset any previously set font map |