Cesium throws an exception if accessor.min and max are missing for POSITION

[lasalvavida]

In glTF, accessor.min and accessor.max are optional properties.

If a model is missing these on an accessor that defines POSITION, Cesium throws:

DeveloperError: array is required
Error
at new DeveloperError (http://localhost:8080/Source/Core/DeveloperError.js:44:19)
at Function.Cartesian3.unpack (http://localhost:8080/Source/Core/Cartesian3.js:174:19)
at computeBoundingSphere (http://localhost:8080/Source/Scene/Model.js:1080:55)
at Model.update (http://localhost:8080/Source/Scene/Model.js:3286:36)
at PrimitiveCollection.update (http://localhost:8080/Source/Scene/PrimitiveCollection.js:366:27)
at updatePrimitives (http://localhost:8080/Source/Scene/Scene.js:1859:27)
at executeCommandsInViewport (http://localhost:8080/Source/Scene/Scene.js:1817:9)
at updateAndExecuteCommands (http://localhost:8080/Source/Scene/Scene.js:1716:17)
at render (http://localhost:8080/Source/Scene/Scene.js:2052:9)
at Scene.render (http://localhost:8080/Source/Scene/Scene.js:2090:13)

[pjcozzi]

Good catch, thanks @lasalvavida. I am tempted to update the glTF 1.0.1 spec to say that min and max are required when the accessor is referenced from a POSITION (or require them always); otherwise, any implementation that uses bounding volumes (which is likely all non-trivial ones) has to have a fallback to explicitly compute it. I opened KhronosGroup/glTF#593 to discuss.

[pjcozzi]

The 2.0 spec will require accessor min and max. Leaving this open until the online converter supports this.