Bullet shape instancing optimization
We should use the btScaledBvhTriangleMeshShape to scale a collision shapes, avoiding redundant copies of the vertex data.
Bullet versions earlier than 2.83 had a bug in btScaledBvhTriangleMeshShape that prevented us using it. When compiled with Bullet 2.83, OpenMW will now take advantage of btScaledBvhTriangleMeshShape.
(RM-2750 from redmine: created on 2015-06-30 by scrawl ., , closed on 2015-06-30 by scrawl .)