Fix 2d only object generation so objects end up in the middle of the z region instead of at the z minimum.
Fix 2d only object generation so objects end up in the middle of the z region instead of at the z minimum.

sirikata/sirikata,sirikata/sirikata,sirikata/sirikata,sirikata/sirikata,sirikata/sirikata,sirikata/sirikata,sirikata/sirikata,sirikata/sirikata