shading issue with i3dm 3dtiles since IBL

1. A concise explanation of the problem you're experiencing.

shading issue with i3dm 3dtiles, since IBL the shading changes in comparison to b3dm with the same materials.

as seen in the sandcastle, with 1.55 there is a shadow at the corners of the cubes

2. A minimal code example. If you've found a bug, this helps us reproduce and repair it.

3. Context. Why do you need to do this? We might know a better way to accomplish your goal.

i think its a bug. same issue when i create my own b3dm and i3dm examples with the same pbrMetallicRoughness material.

4. The Cesium version you're using, your operating system and browser.

cesium 1.55, ubuntu nvidia 1050, chrome v71 and firefox

here i provide the tileset i created. maybe it will help to debug. (2.85 KB)

Thanks for opening the GitHub issue!

This is most certainly a bug.