7997e4aa21
This introduces the `viewer::Settings` struct, and a JSON reader / writer. This will be used for automated testing and for client / server communication. Note that `viewer::Settings` is closely associated with the `filament::View` API; when updating the latter we will often need to update the former, as well as some serialization code. This increases the maintenance burden and I think we should consider using a parser library like libclang or a macro-based reflection utility. This PR also migrates SimpleViewer into libs/viewer and un-inlines its implementation. It does not belong in gltfio because it has an imgui dependency.
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