Bringing back old-style com_fixedTic, adding Carmack's reverse as a compile option

These commits bring back the pre-patch behaviour where setting com_fixedTic to -1 unlimits the framerate while still somewhat keeping everything in sync. (com_fixedTic 1 would make the physics engine go super fast in addition to the framerate, and this is undesirable.) This should help people with, say, 120Hz monitors.

Another feature that was added was the option to compile the engine to use Z-fail in its shadow rendering. This is off by default.