Resize images for thumbnails more naïvely.

* Draws thumbnails true to their aspect ratio and also fixes an issue where the indexed bitmap image returned from the fulcrum scanner could not be scaled down.
* Remove now unused third-party scaling code.

In the future, it might be a good idea to look into just doing a good-old matrix scaling transformation (CIImage iOS 5 and later), but we would need to beef up our image format handling (no indexed bitmap support natively, for instance).