System layer is greatly simplified and framecap
logic has been moved internally. I_Sleep now
takes a sleep duration and I_SleepDuration
generically implements a precise sleep with spin
loop.
Time is now tracked internally in the game using I_GetPreciseTime
and I_UpdateTime. I_Time now pulls from this internal timer. The
system code no longer needs to keep track of time itself.
This significantly improves frame and tic timing in interp mode,
resulting in a much smoother image with essentially no judder at
any framerate.
