Auth plugins with custom signup forms may not be using renderables / renderers - or
even if they are they may return a renderable that can only be rendered with the renderer from the auth
plugin.
This change checks if the signup form is a renderable - if so try the plugin renderer or fall back on the general renderer.
Otherwise call display() from the mform which is the previous way of rendering an mform.
* get_identity_providers(): Retrieves available auth identity providers
* prepare_identity_providers_for_output(): Prepares auth identity
provider data for output (e.g. to templates, WS, etc)
* Use these helpers for the login renderer
Mihail Geshoski