Set the DOMPDF instance in Font_Metrics class.

Avoid error like `file_get_contents(/assets/my_font.woff): failed to open stream: No such file or directory` in `vendor/dompdf/dompdf/include/font_metrics.cls.php`.
Requirement: the pull request https://github.com/dompdf/dompdf/pull/796 need to be merged in DomPDF repositiory.