[pangofc] Add a "fontmap" property to PangoFcFont

[pangofc] Add a "fontmap" property to PangoFcFont

Gecko uses its own PangoFcFontMap subclass with its own PangoFontSet.
Previously we were setting font->fontmap in our own private
PangoFcFontSet.  Now it's up to the PangoFcFont subclass to set it
when creating the new font object.

Also adds the following backend-public symbol:

	pango_fc_font_map_find_decoder()