This feature adds one new method to `FilamentAsset` and uses it in our
Kotlin, JavaScript, and C++ helpers:
utils::Entity popRenderable() noexcept;
This pops a ready renderable off an internal queue, or returns 0 if no
renderables have become ready. It provides a simple way for clients to
gradually add renderables to the scene as they become ready. Previously
clients could only get the entire list of entities, regardless of
whether they had Renderable components or complete textures.
To facilitate this feature, this PR adds a new internal-only class to
gltfio called `DependencyGraph`, which is a temporary object used for
bookkeeping during the asynchronous load.
`DependencyGraph` discovers ready-to-render entities by tracking the
textures that each entity depends on. This is a graph because
renderables connect to a set of material instances, which in turn
connect to a set of parameter names, which in turn connect to a set of
texture objects. These relationships are not easily inspectable using
the Filament API or ECS.
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