Fence item model's bars inconsistent with placed block

The bars connecting fences are modeled differently between the item and block.

1. The bars on the item are 2 pixels high, while on the block they're 3 pixels high.
2. The lower bar is positioned too low on the item.

To fix the item model, the lower bar needs to be moved up 1 pixel and extended 1 pixel upwards, and the top bar extended 1 pixel downwards.

The model provided in MC-182716 also corrects this.