[ Part of bug 118472, reported by Kailash C. Chowksey ]

When checking for glyphs that should be skipped because
of LookupFlags, the code is typically like:

    for ( numlig = ls->LigatureSet[index].LigatureCount;
          numlig;
          numlig--, lig++ )
    {

      if ( in->pos + lig->ComponentCount > in->length )
        continue;                         /* Not enough glyphs in input */
 
[....]

      for ( i = 1, j = 1; i < lig->ComponentCount; i++, j++ )
      {
        while ( CHECK_Property( gdef, s_in[j], flags, &property ) )
        {
          if ( error && error != TTO_Err_Not_Covered )
            return error;
                                                                          
     
          if ( in->pos + j < in->length )
            j++;
          else
            break;
        }
[...]
        if ( s_in[j] != c[i - 1] )
          break;
      }

      if ( i == lig->ComponentCount )
      {
         [ Apply this ligature ]
      }
    }

The initial length check is an initial approximation and probably
useful as a speedup, but not sufficient because we are skipping glyphs;
also there is an off-by-one 

There are various ways that this could be fixed up. I think the
cleanest would be:

     for ( numlig = ls->LigatureSet[index].LigatureCount;
          numlig;
          numlig--, lig++ )
    {
      if ( in->pos + lig->ComponentCount > in->length )
        continue;                         /* Not enough glyphs in input */
       
     [...]
     for ( i = 1, j = 1; i < lig->ComponentCount; i++, j++ )
      {
       if ( in->pos + j == in->length )
          goto next_ligature;

        while ( CHECK_Property( gdef, s_in[j], flags, &property ) )
        {
          if ( error && error != TTO_Err_Not_Covered )
            return error;
                                                                          
     
          j++; 
          if ( in->pos + j == in->length )
            goto next_ligature;
        }
[...]
        if ( s_in[j] != c[i - 1] )
          goto next_ligature;
      }

    [ Apply this ligature ]

   next_ligature: ;
   }

There are, by a rough count, about a dozen places that need
fixing in ftxgsub.c and a dozen more in ftxgpos.c.