dompdf: Image handling broken when rendering multiple files in a row

With the changed setting of DOMPDF_ENABLE_REMOTE in CiviCRM 4.4, dompdf fails to handle images correctly when rendering multiple files in one PHP invocation, because of a bug in the dompdf image cache.

This issue might actually never occur in core CiviCRM itself, as it only shows when generating multiple PDF files in one PHP invocation. However, our extension does this, and thus becomes unusable with CiviCRM 4.4 – so this is indeed a major regression.

For master, the best option is probably to import a current snapshot of dompdf. (There are many many other important fixes and improvements over the ancient snapshot presently used in CiviCRM...)

For the stable branch, this might be considered too risky I guess – but a fix is urgently needed. Thus I created https://github.com/civicrm/civicrm-packages/pull/40 to pick just the fix for this issue.