main loop: Separate simulation time from true time

main loop: Separate simulation time from true time

This allows simulation and other actions, such as camera movements, to
use different time steps. Now camera will move the same speed
regardless of the simulation time (eg. very low fps scenarios).

Furthermore, the maximum time step value is always respected and won't
be exceeded even though user adjusts the simulation speed to higher.

Signed-off-by: Timo Kokkonen <kaapeli@itanic.dy.fi>