Make `Scene` more robust

[karnkaul]

Currently the construction of a scene is separated from it loading GLTF data and populating its storage. This risks invalid use in between those two points. Either refactor two stage init to a single stage, or ensure the scene is ready to be used after construction / does not attempt to render anything until populated.

AC:

1. Scene is ready to use after construction
2. A GLTF source need not be loaded to use a Scene instance