WooCommerce PDF Product Vouchers 3.12.4

* Tweak - Improve compatibility with Print Invoices / Packing Lists to show voucher information
* Fix - Check if other plugins are loading the Dompdf library used by PDF Vouchers to avoid incurring into PHP "Cannot declare class" fatal errors

* Fix - Email single header and subject headings were not used
* Fix - Convert special characters to HTML entities before PDF is generated to avoid potential character encoding issues

* Fix - Voucher templates not properly positioning existing voucher fields when edited
* Fix - New voucher templates not displaying the background image properly
* Fix - Upgrade to 3.1.2 did not finish for sites with more than 50 vouchers
* Fix - Customers unable to view vouchers in the account page introduced in 3.1.1