Adds FC_SYMBOL.

This affects fonts having a cmap with platform 3 encoding 0.
We now map their glyphs from the PUA area to the Latin1 area.

See thread "Webdings and other MS symbol fonts don't display"
on the mailing list.

Test before/after with:
$ pango-view --markup --text='<span fallback="false">&#xd7;&#xf0d7;</span>' --font=Wingdings