Dynamic PDF height using mPDF engine

[alexmigf]

A customer asked us to support dynamic height for the mPDF extension. This is required when printing PDF on a thermal paper roll printer.

We should follow the same procedure that we are doing with DOMPDF here, providing a filter and a code snippet to allow this, instead of including it directly in the plugin.

The PR attached to this issue includes a filter that can be used to dynamically set the PDF height. Here is the code snippet to set the paper width to 62:

add_filter( 'wpo_wcpdf_paper_format', function( $paper_format, $document_type ){
	if ( $document_type == 'packing-slip' ){
		$paper_format = [ 62, 600 ];
	}
	return $paper_format;
}, 10, 2 );

add_action( 'wpo_wcpdf_custom_styles', 'wpo_wcpdf_mpdf_styles', 99, 2 );
function wpo_wcpdf_mpdf_styles ( $document_type, $document ) {
	if ( $document_type == 'packing-slip' ){
		?>
		@page {
			margin-top: 1cm;
			margin-bottom: 1cm;
			margin-left: 0.25cm;
			margin-right: 0.25cm;
		}
		.order-details .total {
			width: 25%;
		}
		.order-details .quantity {
			width: 16%;
		}
		<?php
	}
}

add_filter( 'wpo_wcpdf_after_mpdf_write', function( $mpdf, $html, $options, $document ) {
	if ( $document->get_type() == 'packing-slip' ) {
		$height = $mpdf->y;
		unset( $mpdf );
		$mpdf = new \Mpdf\Mpdf( $options );
		$mpdf->_setPageSize( [ 62, ceil( $height ) + 10 ], $options['orientation'] );
		$mpdf->WriteHTML( $html );
		$mpdf->page = 1;
	}
	return $mpdf;
}, 10, 4 );