… function gimp_font_get_pango_font_description().

Also updating file-pdf-save which is the only plug-in using these right now.

Note that I am not fully happy with the new function
gimp_font_get_pango_font_description() because I experienced some weird behavior
in file-pdf-save which is that some fonts were wrong if this is called after
pango_cairo_font_map_set_resolution().
But let's say this is a first step looking for improvements.