Improve SFNT driver lookup efficiency

Improve SFNT driver lookup efficiency

* src/fontset.c (fontset_find_font): Add compatibility test to
registry strangeness case.
* src/sfnt.c (sfnt_read_cmap_table): Don't read subtable data if
DATA is NULL.
* src/sfntfont.c (struct sfnt_font_desc): New field `registry'.
(sfnt_registry_for_subtable): New function.
(sfntfont_identify_cmap): Move above sfnt_grok_registry.
(sfnt_grok_registry): New function.
(sfnt_enum_font_1): Call it.
(sfntfont_registries_compatible_p): New function.
(sfntfont_list_1): Check registry compatibility.
(sfntfont_registry_for_desc): New function.
(mark_sfntfont): Mark desc->registry.