[ODE] slider2 & other patches

[Geoff Carlton]

For anybody interested in the slider2 joint I described earlier, I put 
the files up on my webpage, at www.zenohm.com.  There is also a version 
of dTerrainZ that accepts a height callback function, and has the aabb 
fix I described earlier.

As to the quickstep warning fix, its a niggly point but surely REAL(x) 
would be preferable to (dReal)x, as the first one creates the constant 
as the proper type, rather than doing a cast.

Geoff