USD Import: material blend bug.

Fixed bug setting the material blend mode due
to an uninitialized variable.

USD Import: material conversion minor updates.

Added comment re UsdPreviewSurface conversion. Now handling
lowercase raw color space specification.

USD import Set Material Blend option.

New Set Material Blend option which, if enabled together
with the UsdPreviewSurface option, will automatically set
the material blend method based on the shader's opacity
and opcityThreshold inputs.

USD material importer comments.

Also updated to more descriptive function name.

UsdPreviewSurface material import improvements.

Now setting texture image color space and chaining a Normal
Map node with a Texture Image node for the Principled BSDF
Normal input. Added a check to print warnings on missing
texture files. Added comments.

USD import: update instancing option description.

USD import: uniform interpolation normals.

Added logic to import USD normals with uniform interpolation
(i.e., per-face normals) as Blender loop normals.

USD import: format fix and remove debug prints.

USD Import: support instance collections.

Extended the experimental instancing option to support
collection-based instancing. (Previously, the initial instancing
experiment was an optimization based on shared meshes, to
reduce memory usage for large instanced scenes, but this did
not conform to the standard Blender instancing workflows and
could not be easily extended to animations.)

USD Importer: typo in code to filter by purpose.

USD Importer: dependency graph update crash.

Fixed occasional random crash due to simultaneous update
to the dependency graph from multiple threads. A typical
collision might happen when the USD import job thread
is creating a material while the window manager in the
main thread is processing event notifiers. Specifically,
invoking BKE_object_material_slot_add() in the import
thread would crash in the call Depsgraph::find_id_node(),
presumably because the call to BKE_scene_graph_update_for_newframe()
from the main thread is invalidating Depsgraph::id_hash.
The current fix is to remove the ND_FRAME flag from the call
to WM_jobs_timer(), which prevents ED_update_for_newframe()
from being called in wm_event_system.c, line 506.
We should look into whether we can skip invoking WM_jobs_timer()
from the import thread altogether.

USD importer: handle 'normals' primvar.

This fix was provided by Koen Vroeijenstijn. Added logic
to read the normals from 'primvars:normals', if this
primvar exists.

USD importer Purpose parameters.

Added logic to prune prims by purpose. UI now has
toggles to enable importing guide, proxy and render
purpose geometry. Minor format fix.

USD importer undo bug workaround.

Skipping call to ED_undo_push() at end of the import,
as invoking this function appears to cause occasional
crashes. This logic should be restored once the
source of the crash is determined.

USD Importer Xform merge bug fix.

Disallow merging if parent isn't a UsdGeomXform.

USD importer: material assignment bug fix.

Added logic to avoid assigning material indices
to instanced meshes more than once.

USD Importer: material assignment improvements.

Simplified code in USDMeshReader::assign_materials() and
added logic to optionally allow assigning mesh material indices,
object material slots, or both.

Fixed USD Import error message.

USD importer: format and compiler warning fixes.

USD importer: import shapes as empties.

Added support for importing USD shapes (e.g., sphere, cube, etc.)
as Blender empty objects, to allow visual debugging of
transforms. Eventually, this logic will be extended to create
meshes corresponding to the actual shapes.