Makes our FT-backed hb_font_t safe to use from multiple threads.  Still,
the underlying FT_Face should NOT be used from other threads by client
or other libraries.

Maybe I add a lock()/unlock() public API ala PangoFT2 and cairo-ft.
Maybe not.

How come this one is not generated by clang everything bot?!

../../../test/api/test-multithread.c:37:26: warning: initialization discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
 static char *font_path = "fonts/Inconsolata-Regular.abc.ttf";
                          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../../test/api/test-multithread.c:38:21: warning: initialization discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]

As Khaled's suggestion, hard-coded font paths was only for my own testing.

Add the default font path used by FreeBSD ports.

As it may cause a race unrelated to the issue actually happened

What were you thinking? ;)