Powder snow will only slow down the player if the bottom of their hitbox is inside of it

The Bug

If only your head is in the powdered snow or you are standing on a block that is not full height then the powdered snow does not slow you down.

Steps to Reproduce

1. Build the setup as shown in setup.png
2. Walk inside of the powder snow and as you do this, pay close attention to your movement speed.
3. Take note as to whether or not powder snow slows down entities if their feet aren't directly inside of it.

Observed Behavior

Powder snow doesn't slow down entities if their feet aren't directly inside of it.

Expected Behavior

Powder snow would slow down entities even if their feet aren't directly inside of it.

Code analysis

Code analysis by [Mod] Avoma can be found in this comment.