Jolt: fix Compound shapes not initializing their _shape

[twhlynch]

Need to look into this further to be sure, but pretty sure JoltCompoundShape would simply not initialize a _shape.
Maybe just adding the super constructor would also fix it.

[twhlynch]

on further investigation, this is the correct fix

[Copilot]

Pull request overview

Fixes Jolt compound shapes so they always initialize their underlying Jolt shape instance (_shape) before children are added.

Changes:

• Initialize _shape in JoltCompoundShape() by constructing a JPH::MutableCompoundShape and AddRef()-ing it.
• Initialize _shape similarly in JoltCompoundShape(const Array *meshes, ...) before enumerating meshes / adding children.

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