GPUParticleSystem serialization and IBL Shadows

[julien-moreau]

• Add worldOffset and preventAutoStart into serialization for GPUParticleSystem
• Realized that iblShadowsTexture was serialized but was useless as it's re-created each time ibl post-process is created

[bjsplat]

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[bjsplat]

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[bjsplat]

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