The root cause of this bug is the fact that Bullet shapes
are shared between duplicated game objects. As the physics
object scale is stored in the shape, all duplicas must
have the same scale otherwise the physics representation
is incorrect.
This fix introduces a mechanism to duplicate shapes at
runtime so that Bullet shapes are not shared anymore.
The drawback is an increased memory consuption.
A reference count mechanism will be introduced in a
later revision to keep Bullet shape shared between
duplicas that have the same scale.
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