Setting the FPS to 120 caused the FPS to flicker erratically between
130 & 140 FPS.
This also impacted lower frame-rates with 23.98 playing back at 24.03
FPS on my system, 30 FPS played back at 30.13 FPS.
This problem was hidden by the FPS display rounding to an integer.
Regression in 2.5x series (worked in 2.49).
Resolve by clamping the sleep time in the main event loop so the 5ms
sleep doesn't result in sleeping when timers are scheduled to run.
There is still some visible FPS jitter that can be solved by using a
higher resolution sleep interval but that's out of scope for this fix.
0e2f4046c6