Hitboxes of plants placed on farmland do not adjust correctly

When plants that are often placed on full-size blocks are instead placed on top of farmland, their hitbox does not account for the pixel of extra space below, causing the hitbox to float.

This (as well as MC-104092) could be fixed by introducing a new block state for the affected plants, which shifts the model downwards by one pixel when the plant is on top of farmland, serving a similar function to the block state that fence gates have.