OldMP: Improve Jetpack and Kicking prediction.
Jetpack prediction can be improved by making use of predicted versions of jetpack_on and moving important SK_JETPACK code from P_ProcessSharedKeys to P_SharedInput.
Kick prediction can probably just be fixed by setting the predicted and non-predicted velocity values at the same time during the non-predicted phase, as weapons themselves are not predicted, thus the jitter. EDIT: This is slightly better, but not good enough. Need to investigate further. Probably going to need to predict kickback_pic somehow.