Working Twig-template loading for simple loading

That is, the following styles work:
* SimpleSAML_XHTML_Template($config, 'hostnames.php');
* SimpleSAML_XHTML_Template($globalConfig, 'negotiate:disable.php');

These do not:

* SimpleSAML_XHTML_Template($this->config, 'discopower:disco-tpl.php', 'disco');
* SimpleSAML_XHTML_Template($globalConfig, 'saml:sp/wrong_authncontextclassref.tpl.php');
* new SimpleSAML_XHTML_Template($config, 'metadata.php', 'admin');
* SimpleSAML_XHTML_Template($config, 'consentAdmin:consentadmin.php', 'consentAdmin:consentadmin');

Loading from themes also not in place