This enables accelleration of heightfield collisions.
Unfortunately, `btHeightfieldTerrainShape::processAllTriangle` does not
yet use the accellerator data, so this change does not improve
performance yet but might do so in future bullet versions.
References:
* Accellerator introduced in:
https://github.com/bulletphysics/bullet3/pull/2062
* Feature request to use the accellerator in `processAllTriangle`:
https://github.com/bulletphysics/bullet3/issues/3276