In the xlib-specific code for cairo_surface_create_similar, there is a code path
that simply returns a newly created image surface.  This means the newly created
surface loses default font options of the screen.

In the generic code for create_similar too, if the backend doesn't support
creating similar, an image surface is created.  Again, the default font options
of the surface will be lost.

A solution may be to have an options font_options in image surfaces that would
be set in this situations.  It may make sense to have somthing like
cairo_image_surface_set_default_font_options () then.