hook_field_attach_view_alter() is unusable

When fired from field_view_field, then the context does not contain anything useful regarding the view mode:

    // Invoke hook_field_attach_view_alter() to let other modules alter the
    // renderable array, as in a full field_attach_view() execution.
    $context = array(
      'entity_type' => $entity_type,
      'entity' => $entity,
      'view_mode' => '_custom',
    );

The patch adds a 'display' key and passes through the $display passed to the function (with defaults filled in). Easy! The semantics of this is "can be either the name of a view mode or an array of display settings" so for consistency we can pass in view_mode in field_attach_view and keep the semantics. Consistency++

Test shows one use case. The real life use case was a bit more complicated: i was rendering entity type A which related to entity type B and both had the same field attached and I needed the field value from B instead of A in some cases and was standing there baffled when I saw _custom.

This patch largely obsoletes the view_mode key/value in the context array but it's too late to change that and luckily the argument to field_view_field is called display and not view_mode so that's a good excuse too why added a new one instead of changing view_mode...