Dropped rings, tossed flags, and other objects will gravflip with the player that dropped them

For some bizarre reason, objects such as dropped rings and tossed flags check constantly for their target's gravity flip status. If the target is flipped, these objects will flip.

This check should be moved out of P_GetMobjGravity entirely, and it should only be done one time when the mobj is spawned from the player.

Relevant discord discussion: https://discord.com/channels/357945222111428609/569169328386211883/879892304570241144

Relevant code: https://git.do.srb2.org/STJr/SRB2/-/blob/next/src/p_mobj.c#L1481