Reinstate the old global cache code. For the forseeable future, it's

Reinstate the old global cache code. For the forseeable future, it's
    probably all right to use the global cache as it was previously and
    just store filenames and font info, as long as no mmap cache exists in
    the directory. Of course, if an mmap cache exists, use that instead.
If a directory cache does not exist or is invalid, load the fonts for just
    that directory using the old codepath.
Fix premature free of the FcPatterns belonging to the FcFontSet which we
    create from the mmapped files.