Support for Apple's package stand-alone USD (v18.09) libs

Kacey,

Currently you will need to initially have USD v18.09 installed on your system and have the Python modules built Pillow (Python module for image manipulation)

Would gltf2usd build with Apples new package python-usd-library ?

In the python-usd-library readme file it stated the following

This binary library was compiled using version 18.09 of [the public USD Github repository](http://openusd.org) with the following build script arguments (see USDPython/README.md for further details):

    python USD/build_scripts/build_usd.py --build-args TBB,extra_inc=big_iron.inc --python --no-imaging --docs --no-usdview --build-monolithic USDPython

[wave-electron]

@enzyme69 @kcoley I’ve undercovered a really strange bug. A png image file named basecolor.png used for my base_map PBR texture for some reason was making all my usdz models that uses that png file crash in ios12 beta 6 onwards. By merely replacing that png file but keeping everything else the same I fixed all those corrupted usdz files. It doesn’t seem to have anything to do with the naming or referencing - but purely the png file! If I can work out what it is I’ll let you guys know.

This was the command line I used to create the file. It was an animated alembic file exported from Blender… which BTW was working perfectly fine up until ios12 beta 6.

xcrun usdz_converter newtons_cradle.abc newtons_cradle.usdz -g base -color_map basecolor.png -metallic_map metallic.png -normal_map normal.png -roughness_map roughness.png -g cradle -color_map basecolor.png -metallic_map metallic.png -normal_map normal.png -roughness_map roughness.png -a -l -v 

This is the usdz file which is now working again in ios12 beta newtons_cradle.usdz

[wave-electron]

Also bouncing ball USD animation from Apple example does not work as USDZ, but maybe it’s related to this bug.

@enzyme69 Apple has some samples in the standalone USD library. For example if you run the python script skinnedMeshAnimation.py You get the skinnedMeshAnimation.usd and skinnedMeshAnimation.usdz in samples/assets directory. These usdz animations are working & i’ve posted skinnedMeshAnimation.usdz on my fusionar.app gallery. Its a bouncing cube with morph animation.