surface: Tank now hit the ground correctly.

Tank uses getHeight method to determine height of the standing point
in the heightfield.  No longer depending on Bullet's rayTest.
This way we can fine tune detection algorithms (which can prevent the
Tank from accidentally penetrate the surface), and more importantly,
customize response to hit event.
surface: Greatly improved getHeight() efficiency of TIN.
surface: Cleaned up TIN::getHeight()'s parameters.