[ODE] continuous forces w/ constant step size

[Marco Grubert]

> Here's the problem with it: The continuously-applied
> forces and the input only get updated once per frame,
> not once per time step, which yields different results
> depending on the frame rate.  These things need to get
> applied per time step to get consistent results.

You're running into two issues here:
a) Continuously applying a fixed force/torque over several substeps making 
up a display frame
b) Adapting the amount of force/torque during a display frame

In my implementation I only care about a) and have added 2 functions to 
set/get the force and torque vectors inside a dBody. Basically before going 
into the WHILE loop I store the forcetorque accumulators in an array for 
each body, and before the end of each loop I reset the accumulators to these 
stored values (since otherwise dStepWorld resets them to 0).

- Marco