I3dm misc fixes

[timoore]

This PR addresses two bugs first reported in the forum post https://community.cesium.com/t/i3dm-transformation-and-textures/32757 :

904 - the tileset glTF up axis was not passed to the glTF converters, but i3dm translation depends on it;

905 - the relative pathnames in external .glb files were not resolved correctly in the i3dm converter.

Fixes #904 Fixes #905 Fixes #893 Fixes #916

[kring]

@timoore this is segfaulting while running the tests, both on CI and on my own system.

[timoore]

Looks good to me! Just one comment in answer to a comment you added in the code.

Regarding the segfault, my guess (but it's only a guess) is that it's related to #893.

Bingo.

[j9liu]

@timoore Let us know when this is ready for a re-review!

[timoore]

@timoore Let us know when this is ready for a re-review!

It's ready for review.

[csciguy8]

Assigning this back to @kring for last checks

[kring]

Thanks @timoore!