Fix issues with gravflip of dropped items (resolves #661)

Removes some code from P_GetMobjGravity that flips dropped items if their target enters reverse gravity.

Adds new code to P_PlayerFlagBurst, P_PlayerEmeraldBurst, P_PlayerWeaponPanelOrAmmoBurst, and P_PlayerRingBurst that applies MF2_OBJECTFLIP if dropped by a player with MFE_VERTICALFLIP