Sync is now sufficiently smooth

Conflating commands with TICs was causing TICs to be skipped if multiple
commands were run serverside, causing jerkiness

N_TryRunTics now returns true if a TIC elapsed and D_Display needs running

Clang warns about D_abs on an unsigned int having no effect, but without
it in r_segs.c, renderer corruption occurs in software mode; re-added
A little cleanup