Gets a default `PangoCairoFontMap` to use with Cairo.
Note that the type of the returned object will depend on the particular font backend Cairo was compiled to use; you generally should only use the `PangoFontMap` and `PangoCairoFontMap` interfaces on the returned object.
The default Cairo fontmap can be changed by using [method@PangoCairo.FontMap.set_default]. This can be used to change the Cairo font backend that the default fontmap uses for example.
Note that since Pango 1.32.6, the default fontmap is per-thread. Each thread gets its own default fontmap. In this way, PangoCairo can be used safely from multiple threads.
the default PangoCairo fontmap for the current thread. This object is owned by Pango and must not be freed.