use the old data structures for single precision serialization, to make it backwards compatible
So files created by the new double precision serialization of Bullet 2.82 cannot be read by old Bullet versions,
but double precision serialized file generated by older Bullet versions can be read by Bullet 2.82 or newer.

git-svn-id: http://bullet.googlecode.com/svn@2668 08e121b0-ae19-0410-a57b-3be3395fd4fd