The glTF.scene should be optional

[javagl]

According to the specification, the glTF.scene field is optional (i.e "not required").

But there are at least two places where gltf.scenes[gltf.scene] seems to be accessed directly:

• https://github.com/AnalyticalGraphicsInc/cesium/blob/43ca55b3693ee1b20fc731813d427a9e8040b758/Source/Scene/Model.js#L3204
• https://github.com/AnalyticalGraphicsInc/cesium/blob/43ca55b3693ee1b20fc731813d427a9e8040b758/Source/Scene/Model.js#L1229

(others may exist - I did not systematically search for all possible occurrences).

When trying to load a glTF without a scene property, this obviously leads to the error

TypeError: t.scenes[t.scene] is undefined

(This basically applies to all the tutorial models. Right now, it can be tested quickly at https://cx20.github.io/gltf-test/examples/cesium/index.html?category=tutorialModels&model=SimpleTexture&scale=1&type=glTF-Embedded-buffer )

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(BTW: The linked lines also raise the question of whether cesium would be capable of handling multiple scenes, but this may be another issue...)

[pjcozzi]

CC KhronosGroup/glTF#815

[javagl]

Seems to be resolved 👴