Fix FPS bug on Windows

[Meoo]

This problem is known in SFML, and seems to come from the resolution of the sleep function Windows use.

It causes random variations of the FPS count depending on the other running applications (try closing / opening your web browser and check the FPS counter).

Possible solutions :

• Use custom active sleep (with tinythread yield) for short times
• Find a way to fix it in SFML

VSync is also affecting the FPS counter...

[Meoo]

Log example

FPS : 60.1868
FPS : 60.0885
FPS : 60.0019
-- Closing Firefox
FPS : 35.9161
FPS : 32.1892
FPS : 35.7981
-- Reopening Firefox
FPS : 49.7906
FPS : 49.9937
FPS : 50.0399

[Meoo]

Seems to be fixed with SFML/SFML#475