Fix warp thread bug caused by Flash 30 perf "fix"

[cwillisf]

The recent changes made in #1396, intended to fix performance problems on Flash version 30, also introduced a timing problem in Scratch threads running in warp mode. Timekeeping was wrong in such cases, leading to incorrect timer block results (see #1397) and causing minutes-long or even permanent hangs in some projects.

This change resolves #1397 by correcting the timekeeping for warp-mode threads, removes the potential for hangs, and also simplifies the thread iteration count estimation math.

I suggest testing this code against at least these three projects:

• 153778200 Natura's Lake / Natura's Adventure
  • Try to jump out of the water on the first screen with an enemy fish.
  • Pass: noticeable hitches.
  • Fail: freeze for several seconds / minutes / forever.
• 10125047 Lag test
  • Pass: project runs, but maybe feels a little slow.
  • Fail: project hangs or runs extremely slowly.
• 227384326 Dungeon Glory
  • Pass: you can wander around the dungeon at an acceptable framerate.
  • Fail: project hangs or runs extremely slowly.

CC @TheLogFather

[TheLogFather]

Any chance this could get bumped? –There have been a number of cases of 'random' crashes and issues reported in the forums due to this. Here are a few I noticed, for example: https://scratch.mit.edu/discuss/topic/307677/ https://scratch.mit.edu/discuss/topic/312487/ https://scratch.mit.edu/discuss/topic/305171/ https://scratch.mit.edu/discuss/topic/312269/ https://scratch.mit.edu/discuss/topic/313729/ https://scratch.mit.edu/discuss/topic/313993/ (And there are numerous occasions I've had to explain to someone why many of my speedtest projects are giving zero for their timings, as well as why a few of my projects crash because they try to use the timer to know when to leave non-refresh.)