Separate shouldRiderSit() from isRiding()

Removing shouldRiderSit() test from Entity.isRiding() would enable mods to render the player 'standing' when mounted on another entity. In my parachute mod the player would appear to 'hang' from the parachute and then when near the ground would pick up legs (sit position) and then land. A hover board mod would probably have the player standing on the board.

In 1.6.2 the coupled tests in Entity.isRiding() causes motion packets not to be sent if shouldRiderSit() returns false. The code is in EntityClientPlayerMP.onUpdate().