Update `forEachTextureInMaterial` to account for extension textures

[jjhembd]

Some of the newer glTF PBR extensions define additional textures. For example,

• KHR_materials_specular
• KHR_materials_anisotropy
• KHR_materials_clearcoat

These will need to be accounted for in forEachTextureInMaterial.

CC CesiumGS/cesium#11970