fix!: replace the gSecondsSinceLastFrame_WorldTime use by the
gSecondsSinceLastFrame_RealTime
This is quite a conceptual change: the physics are calculated not
depending on what we think is the inner game clock, but depending on what we
think is the external real clock. Theoretically this should be a worse choice,
as we calculate physics for the game world with its own world clock. But the
preliminary tests show a better stability of the physics simulation when the
game time is strongly slowed, when the fps are high (for example 100+), and of
course with a combination of both. My hypothesis for this better physics
simulation is that the inner game clock has a larger error margin / a
larger deviation than the real world clock, and that this causes
artifacts in the physics simulation.
Players tests will indicate if this is a choice to keep.