Horse model in inventory doesn't follow mouse cursor correctly if horse is facing south

The bug

The horse model in the inventory of a south-facing horse changes its rotation suddenly rather than smoothly as the mouse cursor is moved horizontally across it. This also affects donkeys, mules, zombie horses and skeleton horses. Llamas are not affected.

Originally reported by Brian Entei. Prior to 1.15, this issue occurred with horses facing north rather than south.

How to reproduce

1. Summon a tamed horse equipped with a saddle:

   /summon horse ~ ~ ~ {Tame:1,SaddleItem:{id:saddle}}

2. Right-click on the horse to ride it
3. Turn to face south (within a few degrees)
4. Open the horse's inventory
5. Slowly move the mouse cursor horizontally across the horse model in the inventory
   → The horse model rotates suddenly rather than smoothly as the mouse cursor crosses the centre