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.

USD import params help text formatting.

USD import flags.

Added Is Sequence and Transform Cache Constraint
import options.

USD transform cache import bug fix.

Fixed USD_get_transform() to work for parented objects.

USD importer: transform cache constraint.

Port of Tangent Animation's implementation of transform
cache constraint for USD (adapted and modified to fit
this version of the importer).

Restored ref counting for USDReaderPrim.

Added back manual ref counting for USD readers, since
we now need this for implementing constraints, and
since it doesn't appear that we will be transitioning
to shared pointers in the near future.

Fixed compiler warning.

Format fix.

USD importer: instancing improvements.

Added new USDDataCache class for caching prototype mesh data.
No longer using a global static cache for prototype meshes.
Now precomputing the prototype meshes in a USDDataCache
instance which is passed as an argument to
USDXformableReade::create_objects(). These changes simplify
the code, provide better support for thread safety and circumvent
potential issues with dangling pointers in the previous
implementation.

Removed debug output.

UsdPreviewSurface shader network import.

Added experimental code to import UsdPreviewSurface shaders
as Blender Principled BSDF node networks.