ClassCastException when receiving vanilla MovingObjectPositions

[falkreon]

The reduceMOP function in BlockMultipart blindly assumes that the MovingObjectPosition is an ExtendedMOP. This is a bad assumption.

We got word of the problem originally over here: https://github.com/elytra/FruitPhone/issues/13

Expected: Calling BlockMultipart::getPickBlock in a manner identical to vanilla produces the same result as if the block were picked.

Observed: Client crashes.

Known Workarounds: Crash is only known to occur in interactions with specific mods, so removing either FMP or the mod it's interacting with is sufficient to stop the problem.