Invoice PDF generation with php7

When trying to generate invoice pdfs using php 7, my php is errorring out with memory issues, even when I push it to 1024 Mb.

Downgrading the php version to 5.6 fixes it.

I note that I get a lot of warnings like this:

Warning: A non-numeric value encountered in Dompdf\Css\Style->get_computed_border_radius() (line 1326 of /var/www/civicrm/4.7/vendor/dompdf/dompdf/src/Css/Style.php).

when using php7, which I don't get with php5.6, so perhaps dompdf has an issue.

I note that I've configured this test site to use wkhtmltopdf, but judging from the above it would appear that this code doesn't try to use it, and perhaps that would fix it.

Alternatively, it seems that upstream dompdf has been patched, perhaps that just needs to be pulled into Civi core (https://github.com/dompdf/dompdf/pull/1030)

This issue should be a sub-issue of CRM-17789