Add print_pdf_dompdf_alter hook to support dompdf object access

Problem/Motivation

This came up because trying to print pages in my local development environment was failing because Dompdf was not accepting my self-signed SSL certificate for background image URLs and such.

Steps to reproduce

You can see the SSL warnings by setting up the Dompdf global variables:

  global $_dompdf_warnings;
  $_dompdf_warnings = [];
  global $_dompdf_show_warnings;
  $_dompdf_show_warnings = true;

and then inspecting $_dompdf_warnings after the call to $dompdf->render() in print_pdf_dompdf_print_pdf_generate().

Proposed resolution

You can set a HTTP stream context to ignore SSL failures on the Dompdf object. If we add support for a hook_print_pdf_dompdf_alter() hook (similar to the other PDF library handlers), people can do this in their custom modules:

function localdev_print_pdf_dompdf_alter($dompdf) {
  $dompdf->setHttpContext(stream_context_create(array('ssl'=>array('verify_peer'=>FALSE,'verify_peer_name'=>FALSE))));
}

Remaining tasks

I'll attach a patch to add this line.

User interface changes

None.

API changes

Create a new hook_print_pdf_dompdf_alter($dompdf) hook.

Data model changes

None.