Implement USDZ exporter improvements

PR https://github.com/playcanvas/engine/pull/4638 implemented initial functionality of USDZ export, and these are additional bits that would improve it's quality:

• scene hierarchy. At the moment, mesh instances are exported without a hierarchy, each directly with a world matrix. A hierarchy would be useful when animations are exported.

• animation support

• skinning / morphing support

• precision - all mesh attributes (positions, uvs ..) are exported at full precision, and could be stored with lower precision to limit the size. Example:

point3f[] points = [(-33.66764831542969, 3.8385066986083984, 19.865646362304688) ...

• additional vertex attributes (for example vertex colors)

• additional texture formats. All textures are presently stored in .png format. We could add jpg format if alpha channel is not required.

• texture transform
  There is partial support implemented, but does not work in many cases:

                    float inputs:rotation = ${rotation}
                    float2 inputs:scale = (${tiling.x}, ${tiling.y})
                    float2 inputs:translation = (${offset.x}, ${offset.y})

https://github.com/KhronosGroup/glTF/blob/main/extensions/2.0/Khronos/KHR_texture_transform/README.md
https://graphics.pixar.com/usd/release/spec_usdpreviewsurface.html#transform2d

• roughness / glossiness
  at the moment, the texture is assumed to be a roughness (to match gltf specification). In case PlayCanvas material is used in the scene, this most likely is a glossiness, and we could perhaps handle it this way: https://graphics.pixar.com/usd/release/spec_usdpreviewsurface.html#roughness-vs-glossiness

• support additional material settings (clearcoat, ior, sheen) - see more here https://graphics.pixar.com/usd/release/spec_usdpreviewsurface.html#preview-surface

• texture repeat / clamp - at the moment, wrap is hardcoded

                    token inputs:wrapS = "repeat"
                    token inputs:wrapT = "repeat"

• alpha testing - the code is there, see opacityThreshold, but using it seems to fail to load in the built in MacOS viewer. Needs further investigation.