[ODE] auto-disable patch (yet again)

[Russ Smith]

> So, a patch for auto-disabling of bodies, yet again. It's here (watch
> out for long links):

this looks pretty good, so i've checked it in to CVS, and written
documentation:
	http://opende.sourceforge.net/ode-latest-userguide.html

i made a few modifications:
  * it's not conditionalized: it's now always compiled in.
  * the world defaults are a bit different.
  * the auto-disable checking happens in a function in util.cpp, that is
    shared between the slow and fast step functions.
  * i changed the auto-disable code a bit: you might want to verify that
    my changes make sense.

the test_boxstack demo has been modified (a one line change!) to
auto-disable its bodies. disabled bodies display as red ... try it, it's
fun!

note to all: this can be a BIG speed win in certain situations. 

there are two big issues still to resolve with auto-disabling:

  * in a stack of disabled bodies, if one of the bodies is removed (or
    otherwise changed) it should re-enable all bodies in the stack, but
    it doesn't. there is actually a more general problem here: how to
    detect configuration changes for disabled bodies.
  * we shouldn't be doing collision work for disabled bodies.

russ.

-- 
Russell Smith
http://www.q12.org