Crash when loading glTF's KHR_texture_transform without a sampler

[emackey]

Cesium's implementation of the KHR_texture_transform extension of glTF makes poor assumptions that the optional sampler will be defined.

https://github.com/AnalyticalGraphicsInc/cesium/blob/90b130edea858b7b6b6a1575422b0b2654f2ae97/Source/Scene/processPbrMaterials.js#L94

In the above, gltf.samplers need not be defined, and gltf.textures[n].sampler is likewise optional, leading to a rather fancy message:

Cannot read property 'undefined' of undefined

Here's a sample model for testing.

TexRotateTest.zip

[mramato]

So it looks like the behavior is to have repeatS/repeatT be false if there's no sampler?

[emackey]

No, the default wrapping value in glTF is REPEAT, so when not specified, repeat should be true.