Fixed: Font subsetting - incorrect placement of head checksum

[jindrapetrik]

Currently, during font subsetting, head checksum is calculated and placed on offset 8 after head table start,
but this is incorrect and causes overwriting head.offset field!

The checksum should be placed in offset 4 after the tag 'head'.

This PR fixes this.

For me, this fixed PDF display error on Adobe Reader and or not diplaying proper characters in Edge PDF viewer.

[CLAassistant]

All committers have signed the CLA.

[williamdes]

Can you provide a before and after pdf please?

[jindrapetrik]

Yes, here:
before.pdf
after.pdf

Using code like this:

      $pdf = new \TCPDF();
      $pdf->AddFont('Unifont', '', PDF_FONTPATH . 'unifont.php', true);
      $pdf->setFont('Unifont', '', 16);
      $pdf->AddPage();
      $pdf->Write(16, "ABC");
      $pdf->Output('out.pdf', 'D');

(I am using TCPDF 6.4.4, but the problem is probably the same in current.)

[williamdes]

Thanks
@nicolaasuni ready for your review