3D Tiles - Use tileset version when querying external uris

[lilleyse]

For #3241

I combed through the 3d-tiles code for cases of requesting external uris. In some areas the query and fragments of the tileset url would not be used, in particular this means the tileset version would be missing.

I'm about to add some tests as well.

[lilleyse]

Fixes CesiumGS/3d-tiles#187

[lilleyse]

Ready now. The tests check a variety of cases:

• i3dm with an embedded gltf that references external images/shaders
• i3dm with external gltf that references external images/shaders
• b3dm with embedded gltf that references external images/shaders
• Nested tilesets to ensure the query parameters are preserved for external tilesets