Mob position in the air client-server desync

Blazes and Withers will often appear in a spot that they are not whilst flying.

Steps to Reproduce:
1. Make a new Creative world.
2. Spawn a Blaze and hit it.
3. Fly upwards a few blocks and observe it as it hovers upwards.
4. As the Blaze is hovering, notice how it bobs up and down when going back to the ground or just hovering in place.
5. For Withers, refer to MC-9242.