#4 Rewrite vehicle code

[Gordon Tisher]

Redo the vehicle code (gsgl::physics::vehicle and gsgl::physics::vehicle_module).

Currently the entire vehicle (i.e. the sum of its modules) is treated as a single rigid body.

Should be re-written so that each module is a separate rigid body, and the various modules in a vehicle should be joined by constraints.

This will allow large non-convex vehicle shapes (with moving parts).

[Gordon Tisher]

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This obviously depends on request 1905517.