#127 body/joint creation with null world

[rodkay]

• allows the World parameter to be null (0) when creating a body or joint.
• world can later be set with new dBodySetWorld or dJointSetWorld functions.
• the d*SetWorld functions can also be used to remove a body or joint from the world (when arg is null), or to move it intto another world (when arg is not null).

[rodkay]

body/joint creation with null world patch

[Daniel K. O.]

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I don't like the proposed semantics: bodies and joints only have meaning in a world where they can be stepped. Allowing for null worlds means we break an important invariant: the object can now be in a semi-constructed state, where not all operations are valid anymore.