Images get removed after first render

Images used in the PDF (i.e. $pdf->Image($base_path . '/assets/background.png', 0, 0, 210, 297);) will be removed after the first render in some cases (not sure what the conditions are).

There's an issue about this (https://github.com/tecnickcom/TCPDF/issues/158) but the latest version of the TCPDF library doesn't seem to fix the issue. The proposed solution to unset the imageKeys property before destroying does work so I created a patch that adds this solution to this module.