support for 4byte characters

[vavrecan]

OpenSansEmoji.ttf does not work properly

https://github.com/MorbZ/OpenSansEmoji/blob/master/OpenSansEmoji.ttf
I think that 4 bytes characters are not imported properly.

emoji characters e.g. 😃 are rendered as square box.

`
$pdfLibrary = new tFPDF\PDF();

$pdfLibrary->AddPage();

$pdfLibrary->AddFont('OpenSansEmoji', '', 'OpenSansEmoji.ttf', true);
$pdfLibrary->SetFont('OpenSansEmoji', '', 14);

$pdfLibrary->Write(8, "😃");

header("content-type: application/pdf");
print $pdfLibrary->output();
`

can you please help?
Thank you