The limiter slept off the rest of each frame with sleep_for() given a float
duration. MSVC adds that to steady_clock::now() in float nanoseconds, so after
a few hours of system uptime the wake-up time is rounded to tens of ms: at
95 h uptime fpslimit 60 had no effect at all and fpslimit 30 gave ~22 fps, in
the full and the legacy renderer alike.
frame_pacer now schedules frames on a fixed cadence in steady_clock ticks and
the loop waits with sleep_until; an overshooting wait is made up in the next
frame. The FPS shown and the motion blur use the new mainloop_period stopwatch
(frame start to frame start, wait included); the targetfps/fpsaverage
draw-range controllers keep reading the work time as before.
(eu07/maszyna#416)
