OnGround tracking fixes

[sapphire-arches]

Makes OnGround tracking work better. I wouldn't be confident in saying it's 100% correct now though... Fixes:

• Who knew that Vector3.Zero is not in fact greater than Vector3.One
• _position is actually the head position, we need to test the feet position - 1
• Sometimes the game world hasn't loaded when we need to access OnGround
• OnGround should update every time we send it to the server

Update

[ddevault]

Thanks, @bobtwinkles