This is a breaking change needed to fully implement instancing and stencil highlighting.
Previously, users would work directly with entities (on the Dart side, ThermionEntity), e.g.
final entity = await viewer.loadGlb("some.glb");
However, Filament "entities" are a lower-level abstraction.
Loading a glTF file, for example, inserts multiple entities into the scene.
For example, each mesh, light, and camera within a glTF asset will be assigned an entity. A top-level (non-renderable) entity will also be created for the glTF asset, which can be used to transform the entire hierarchy.
"Asset" is a better representation for loading/inserting objects into the scene; think of this as a bundle of entities.
Unless you need to work directly with transforms, instancing, materials and renderables, you can work directly with ThermionAsset.
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