title: NSImage has opacity when used with NSValueTransformer
body: <p>I am trying to show an NSImage depending on a enum value from out of Core Data.
  I am using a Value Transformer on an NSImageCell bound to the value on which I
  choose the image to show.</p>
  
  <p>It works perfectly and as intented, unfortunetaly when using the NSValueTransformer the image gets an opacity it is not supposed to have.</p>
  
  <p>This is an excerpt from my NSValueTransformer subclass:</p>
  
  <pre><code>+ (void)initialize
  {
      downloadingImage = [NSImage imageNamed:@"downloading.png"];
      downloadFailedImage = [NSImage imageNamed:@"download_failed.png"];
      downloadCompleteImage = [NSImage imageNamed:@"download_complete.png"];
  }
  + (Class)transformedValueClass { return [NSImage class]; }
  + (BOOL)allowsReverseTransformation { return NO; }
  - (id)transformedValue:(id)value {
      switch([value intValue])
      {
          case DownloadStatusComplete:
              return downloadCompleteImage;
          case DownloadStatusFailed:
              return downloadFailedImage;
          case DownloadStatusNone:
              return nil;
          case DownloadStatusDownloading:
              return downloadingImage;
          default:
              return nil;
      }
  }
  </code></pre>
  
  <p>The corresponding static NSImage definitions:</p>
  
  <pre><code>static NSImage* downloadingImage;
  static NSImage* downloadFailedImage;
  static NSImage* downloadCompleteImage;
  </code></pre>
  
  <p>and the enum I use:</p>
  
  <pre><code>typedef enum {
      DownloadStatusNone,
      DownloadStatusDownloading,
      DownloadStatusComplete,
      DownloadStatusFailed
  } DownloadStatus;
  </code></pre>
  
  <p>Below is a screenshot from the actual application and one without the NSValueTransformer but a static NSImage assigned in IB. So it's not the PNG that is having an alpha or something. This happens with three different images from distinct locations.</p>
  
  <p><a href="http://imageshack.us/a/img195/2492/appqe.png" rel="noreferrer">actual application http://imageshack.us/a/img195/2492/appqe.png</a>
  <a href="http://imageshack.us/a/img27/3289/43084671.png" rel="noreferrer">static image http://imageshack.us/a/img27/3289/43084671.png</a></p>