Function setSceneImageProcessing

Update a scene's image-processing configuration after registration, rebuilding the pipelines that bake image-processing state into their shaders only when necessary.

Cost tiers:

• exposure / contrast are read live from the scene UBO every frame — changing them takes effect on the next frame with no rebuild.
• toneMappingEnabled and toneMapping are compile-time PBR shader features. Changing either recompiles the affected PBR pipelines (via rebuildScenePbrPipelines).

The rebuild is skipped when the scene has not been built yet (its first build will pick up the new configuration) or when only live UBO fields changed.