Fix #123972: Hydra storm crash with USD sub-d meshes

When using the USD export method with Hydra storm[1], problems can occur
because of how this was integrated alongside the direct Hydra method.
The direct hydra support was initially added in Blender 4.0 and the USD
option was integrated at the same time in order to provide a mechanism
for comparison and double-checking each implementation.

In the context of this bug, for Viewport previews and renders, the Hydra
engine is initially triggered and executed as part of an "engine update"
call from the various v3d draw managers. During this call the USD export
is attempted.

For sub-d meshes the USD export machinery will, by default, attempt to
apply the correct Subdivision Scheme attribute to mesh data. That means
it will export the unsubdivided base mesh with an attribute letting the
downstream receiver of the data know they should do the subdivision on
their own. This subdivision scheme support was added in 4.1.

However, in order to do this, USD must first disable the Subdivision
modifiers in Blender before exporting the mesh. Disabling modifiers
triggers depsgraph processing and, unfortunately, this processing will
also trigger an "engine update" for Hydra... again.

Reentrancy is not supported here. See stack trace in original bug.

So, instead, change the USD export option to output a subdivided mesh to
begin with. This has the following ramifications:

- Viewport material preview and render modes no longer crash when sub-d
  is used

- While F12/final renders did not crash, changing this option will now
  properly render the subdivided geometry when using the USD export
  method. Allowing our USD / Hydra render tests to align more closely.
  The direct Hydra option was already pre-subdividing mesh data anyhow.

  This will require updating the USD reference render images.

- The underlying integration issue is not fixed. Triggering a USD export
  inside the "engine update" call path seems error-prone and can lead to
  similar issues in the future.

Pull Request: https://projects.blender.org/blender/blender/pulls/125840

source/blender/io/usd/hydra/usd_scene_delegate.cc

@@ -55,6 +55,7 @@ void USDSceneDelegate::populate(Depsgraph *depsgraph)
   params.use_instancing = true;
   params.relative_paths = false;  /* Unnecessary. */
   params.export_textures = false; /* Don't copy all textures, is slow. */
+  params.export_subdiv = USD_SUBDIV_TESSELLATE;
   params.evaluation_mode = DEG_get_mode(depsgraph);
   params.generate_preview_surface = !use_materialx;
   params.generate_materialx_network = use_materialx;