Refactor sfntfont.c

Refactor sfntfont.c

* src/sfntfont.c (struct sfnt_font_tables): New structure.
(struct sfnt_font_desc): New field `tables'.
(struct sfnt_font_info): New field `desc'.
(sfntfont_setup_interpreter): Drop fd arguments and don't try to
load interpreter tables.
(sfnt_open_tables, sfnt_close_tables): New functions.
(sfnt_reference_font_tables, sfnt_dereference_font_tables): New
functions.
(sfntfont_open, sfntfont_close): Implement in terms of those
functions in order to share tables.