/**
 * Copies a portion of pixels from the image bitmap, and returns
 * that portion in a new image bitmap.
 * @param nRect The frame from which to cut the new image.
 * If this frame doesn't perfectly intersect with the images bounds,
 * the excess parts of the returned image will be fully transparent.
 * @return An autoreleased image bitmap that has been cropped.  This image
 * will have the dimensions of @param nRect.
 */
- (ANImageBitmapRep *)cropWithFrame:(CGRect)nRect;