Add fluid simulation example

[ttnghia]

Finally, fluid simulation example for magnum 😍
Please make a WebGL example of this. I don't know how to do it now :)
Let me know if there's anything needs to be changed.

[mosra]

Yay, Xmas come early this year! I need to try this out immediately :)

[alanjfs]

The ThreadPool stuff is super interesting. :O Thanks for sharing this!

[alanjfs]

Adding a gif, for easy sharing. :)

• https://vimeo.com/368947792

[ttnghia]

I pushed new changes:

• Add back tbb code.
• Limit 60FPS rendering but maximize computation: run a number of simulation steps (which is dynamically computed) per frame.

[mosra]

Thank you, this is great 👍

Sorry for the lack of action on my side, got a priority thing to work on right now. I'll try to get this merged later this week.

[mosra]

Merged as 4fb94bd and ba5000c -- you did some great work, thanks a lot 👍 I didn't look into the WebGL port yet, hopefully soon :)

Docs for the example are at https://doc.magnum.graphics/magnum/examples-fluidsimulation3d.html -- I was not sure what exactly to put there, so if you have some further information, let me know.