USD IO: handle UMM Python module load error.

Clearing the Python import module error if loading
the UMM module failed. If we don't do this, the
Python unit test for USD will fail if the UMM
addon isn't installed. Also printing the Python
error in this case, if printing warnings is enabled.

USD export: fix build errors with latest master.

Merge branch 'master' of git.blender.org:blender into universal-scene-description

USD IO improved feedback.

Now parsing the Python notification dictionary object
returned by UMM to output warnings and error to the
Blender log.

Removing unneeded log message when setting the
default prim, as it can obscure more important
warnings and errors in the status bar.

USD IO options improvements

Added default values for the Default Prim Path,
Root Prim Path and Material Prim Path export
options. Now validating that these options are
set to well formed USD paths and raising an
error if these paths are invalid. This helps avoid
potential crashes when attempting to define
USD prims with invalid paths. Also updated import
shaders option menu tooltip.

USD Import: minor format fix

Merge remote-tracking branch 'blender_org/master' into temp-usd-latest-master

Merge remote-tracking branch 'blender_org/master' into temp-usd-latest-master

USD export: redundant call to set stage units

Setting the stage meters per unit metadata was being called
in two places unnecessarily. Removed redundant call.

USD import: 'preview' purpose material fallback

Added logic to explicitly query bound materials
with purpose 'preview', if querying 'allPurpse' bound
materials returns no result.

USD IO: material import improvements

UDIM texture support on UsdPreviewSurface import.
New Material Name Collision option for sepcifying behavior when
an imported material name conflicts with the name of an
existing material. Also includes format fixes.

USD IO: fix compiler warnings and errors

Fixed warnings and errors for linux and darwin
builds. Also fixed copyright date in
usd_light_convert.cc.

USD IO: initial commit of extended features

Instancing import: Import USD scene instances as Blender collection instances.

Instancing export: Extend the existing instancing option to support exporting arbitrary object hierarchies as USD scene instances. Additional support for exporting Blender particle systems as USD point instancers.

Environment map IO: Logic to convert between USD dome lights and Blender world materials, including environment textures.

Unit conversion scene scale: Automatically scale the scene for unit conversion on import and export (e.g., scale the imported objects based on the USD?s meters per unit value).

Curve export.

Armature export: Export armatures and skinned meshes to USD skeletons and skeletal animations.

Light unit conversion: Experimental code to convert between light intensity units in Nits and Blender?s light energy units, on import and export.

Transform operator options: Option to save transforms to USD as the combination of scale, rotate and translate operators, where the rotation can be expressed as Euler angles or a quaternion.

Export to USD shader nodes: Convert Blender shader nodes to UsdPreviewSurface nodes, MDL material nodes or a custom USD representation of Cycles shaders. (MDL export requires UMM addon to be installed.)

Import MDL materials: Convert MDL materials to Blender shader networks. (Requires UMM addon to be installed.)

Texture export: An option to save textures to a directory relative to the USD being exported, using either absolute or relative asset paths. This feature works with UDIM tiles as well as packed and in-memory ?baked? textures.

Option to specify a default primitive on export.

Option to add a root primitive on export. This option adds a single prim as the parent of all exported prims.