The Engagement Report tab page renders templates/CRM/WebformMw/Page/Tab.tpl
via Smarty. Without an explicit template-dir registration, Smarty cannot
locate the file and the tab fails to render. Use CRM_Core_Smarty's
prependTemplateDir() (cleaner than splicing template_dir by hand).
Mirrors the same fix the CiviCRM admin applied on the server so the repo
source no longer drifts from the working deployed state.
Joel Brock
57bd5be4ab