This is to make sure than regardless of how many fonts there are, font loading
won't delay startup.
FontConfig if compiled against libxml2 might fail to parse huge xml configs.
So instead of passing it a huge xml, pass the xml every MAX_NUM_FONTS_PER_CONFIG (currently 1000) fonts.
Also made GimpFontFactory custom config strings private because there is no reason for them to be public.
… 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.
