Prevent double processing in fivestar_field_prepare_view (ratings removed)

Problem/Motivation

If field_view_field() is manually used to display the fivestar field and it is called more than once (e.g. because of different settings) the rating values can be removed when the field isn't exposed or it adds unnecessary overhead if the field is exposed.
Example code:

$small_widget = field_view_field('node', $node, 'field_rating', array(
  'label' => 'hidden',
  'type' => 'fivestar_formatter_default',
  'settings' => array(
	'style' => 'average',
	'text' => 'average',
	'widget' => array(
	  'fivestar_widget' => 'my_small_widget.css',
	),
  ),
));

$huge_widget = field_view_field('node', $node, 'field_rating', array(
  'label' => 'above',
  'type' => 'fivestar_formatter_default',
  'settings' => array(
	'style' => 'average',
	'text' => 'average',
	'widget' => array(
	  'fivestar_widget' => 'my_very_huge_widget.css',
	),
  ),
));

Proposed resolution

To prevent this fivestar_field_prepare_view() should check if the field items already were processed and skip processing if so.
The attached patch became bigger since there were quite some trailing white-spaces. My IDE removes them automatically to comply with the Drupal coding standards. If there's a need for a patch without these white space changes I'll provide one.

Remaining tasks

reviews needed.

User interface changes

none

API changes

none