- modified the interface of the "sfnt" module. There is now a function
    called "load_format_tag", and another called "load_directory".

    The first one is in charge of returning the 4-byte tag located at
    the beginning of a given font file. It understand TrueType collections
    and parses them automatically

    The second loads the table directory that is located just after
    the format tag.

    This is useful, because the "SFNT" storage scheme can be used by
    several distinct formats, each with its own format tag.

    The TrueType driver now checks the format tag in "src/truetype/ttobjs.c"

  - made some changes to "src/shared/t1types.h" to clearly separate the
    Type 1 font content from the rest of the T1_Face structure. This
    will be useful when adding the CFF/Type2 driver that will be able
    to reuse the "T1_Font" structure within a "TT_Font" one (which
    really describes a SFNT-based font file).

    Some changes in "src/type1" were thus performed to reflect this.
    Note that the current type1 driver will be discontinued in a
    distant future. More on this later..