[ODE] Contact joint MotionN

[Geoff Carlton]

I've added a new feature to the contact joint to handle object-world 
collisions where the world is moving.  In my case, I have a lift trimesh 
that has no body, but does have a velocity.  To allow this, I've added a 
MotionN field to the contact enum that allows the contact to incorporate 
a velocity along the collision normal (in a similar way to the existing 
Motion1/Motion2 along the other 2 directions).

I haven't got around to a patch file yet since my ode has diverged a bit 
(I've also added the PosR change for offset bodies that was discussed 
previously), however the code is very simple.

The enum in contact.h:
  dContactSlip2        = 0x100,
  dContactMotionN    = 0x200,
  dContactApprox0    = 0x0000,

The contactGetInto2 in joint.cpp:
Calculate motion:
  dReal motionN = 0;
  if (j->contact.surface.mode & dContactMotionN)
  {
      motionN = j->contact.surface.motionN;
  }
Then we apply it to our 'c':
  dReal pushout = k*depth + motionN;
  if (pushout > maxvel) info->c[0] = maxvel; else info->c[0] = pushout;
Bounce is changed to the following:
      dReal newc = - j->contact.surface.bounce * outgoing + motionN;
      if (newc > info->c[0]) info->c[0] = newc;

I'll make up a patch file if anybody is interested (or I get bored and 
want something to do).
Geoff