Have rdf.module only act on renderable entities

Patch

Yeah, getRenderController() and the other getController() methods are not totally ready for what we want to do here.
Raising and catching exceptions in regular request flow (like, here, on every ConfigEntity load...) is something we typically try to avoid.

- We should be able to call getController() and receive NULL/FALSE without having to catch an exception
- Besides, we don't actually need to instanciate the controller here, but merely check that one is defined. So getControllerClass() should have this "NULL / no exception please" ability.

[edit : pinged folks in #1807042: Reorganizie entity storage/list/form/render controller annotation keys about this]

Instead of instantiating the controller, why don't you retrieve the entity info and check whether there's a render controller defined?

Sure, I have a patch that does this - but that is not ideal encapsulation IMO. Dealing with this should be the job of the EntityManager (like, #1807042: Reorganizie entity storage/list/form/render controller annotation keys is about to reshuffle the whole structure of the 'controller classes' within the info).
getControllerClass() *almost* does what we need, we just need a "no exceptions" behavior.

Yup, rdf_entity_load() populates $entity->rdf_mapping, but the data is only actually used in implementations of hook_preprocess_[entity]() or hook_field_attach_view_alter(). So loading it in hook_entity_prepare_view() would make sense.

One exception though : rdf_comment_load() uses the 'rdf_mapping' property set in rdf_entity_load() :

/**
 * Implements hook_comment_load().
 */
function rdf_comment_load($comments) {
  foreach ($comments as $comment) {
    // Pages with many comments can show poor performance. This information
    // isn't needed until rdf_preprocess_comment() is called, but set it here
    // to optimize performance for websites that implement an entity cache.
    $comment->rdf_data['date'] = rdf_rdfa_attributes($comment->rdf_mapping['created'], $comment->created->value);
    $comment->rdf_data['nid_uri'] = url('node/' . $comment->nid->target_id);
    // ...
}

"set it here to optimize performance for websites that implement an entity cache" :-)
Thoughts ?

Also, even though rdf_entity_load() will be better suited as hook_entity_prepare_view() (and thus will only fire on the right entities), we still need to stop rdf_entity_bundle_info_alter() from loading stuff for non-rendereable entity types.