Content drawn with Cell(), Write(), Text() and the other direct drawing calls is untagged under PDF/UA

[jakejackson1]

On #325 (feature/pdfua-1-support), content drawn with the direct drawing API is not tagged, so the document fails veraPDF ua1 rule 7.1-3: content is neither marked as an artifact nor tagged as real content.

$mpdf = new \Mpdf\Mpdf(['PDFUA' => true, 'PDFUAauto' => true]);
$mpdf->SetTitle('Doc');
$mpdf->WriteHTML('<h1>x</h1>');
$mpdf->Cell(40, 10, 'Text');

The #389 audit found 11 of 14 direct-API documents failing:

• Fail: Cell(), MultiCell(), Write(), Text(), WriteText(), WriteCell(), CircularText(), Shaded_box(), Rect() and Line().
• Pass: AutosizeText(), Image() and WriteFixedPosHTML().

Decision needed: what each call is tagged as.

• Should text calls each become a P, or join the current structure element?
• Should Rect(), Line() and other pure graphics become artifacts?
• Should an alternative be to warn under strict mode, or wrap as artifacts under auto?

Whatever is chosen, add each call to the veraPDF group.