title: gltf model lost its lighting and is dark
body: <p>I have converted a dae file to gltf file, using the Collada converter on the website (I am using Linux, so I couldn't find debugging instruction before and after conversion). When I load the model into the cesium it was too dark. Then I edit the shaders of gltf file and remove 'defuse.xyz=defuseLight' and it looks a little bit better, but it is still dark. I attached pictures of the model in its original format and in cesium(after editing). Can any one help me to see what is the problem?<a href="https://i.stack.imgur.com/y8rAL.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/y8rAL.png" alt="The model as I expected to see"></a>
  <a href="https://i.stack.imgur.com/WpImy.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/WpImy.png" alt="The model on Cesium"></a></p>