Fix performance issues and introduce Performance example

[marcofugaro]

To fix the performance issues I've reintroduced the check on substepping that was removed in 86b0444 and introduced the changes from schteppe#392, which don't let the accumulator grow overtime.

I also introduced a new performance.html example, which I used to do the tests.

When jank is introduced, the simulation should stutter but still have a fixed timestep, it should not play in slow-motion.

When there is too much stuff going on for javascript to handle (500 boxes), I've set the minimum framerate at 10fps 30fps, which is a good compromise between time-correct simulation and playable fps. This can of course adjusted by the user using the maxSubSteps parameters.

(NOTE: the example is rotated until PR #34 is merged and Demo.js is updated)

This should close #16
This should close #36

⚠️ Please test that those fps issues don't happen anymore before merging.

[codynova]

Awesome job hunting this down and fixing it, this is a huge deal as far as features go. 💥💥💥 We should advertise this heavily in the readme. From what I can tell this is working excellently based on my testing in #16 and #36 (and after merging #41). I'm going to release this, there really isn't much risk even if everything were to break.

[marcofugaro]

Thanks! 🙂
Glad to know your tests went okay!!