Make FontMetrics available to inline script as $fontMetrics

Since the FontMetrics class is no longer designed to be used statically a user must instantiate their own copy or reuse the one instantiated by dompdf. While not overly difficult to instantiate
(see load_font.php) there will be less friction and easier migration for the end user if we provide access to the instance used by dompdf.
Re-enable runtime paper size and orientation configuration