Various optimizations and cleanup in the render delegate

[sebastienblor]

Changes proposed in this pull request
The libs/render_delegate/ code has accumulated several correctness bugs and performance inefficiencies. This plan addresses them in priority order: safety first, then hot-path performance, then modern C++ hygiene.

1. Exception-unsafe manual locking in render_buffer.cpp

Map() calls _mutex.lock() then returns early calling _mutex.unlock() manually. Not exception-safe; also, Unmap() conditionally skips unlock() if buffer is empty, creating potential deadlock.

2. Potential null dereference in shared_arrays.h

unboxed.values[0] accessed without checking unboxed.count > 0.

3. Various string optimizations

Several optims on string allocations in render_delegate.cpp , render_settings.cpp, volume.cpp

4. O(n²) light linking in render_delegate.cpp and O(n²) duplicate detection in volume.cpp

• std::find() on std::vector<HdLight*> per registration → O(n) per call → O(n²) total for n lights.
• std::find_if on fields vector per field entry.

5. Use smart pointers to avoid manual new / delete where the destructor must remember to delete

• _imagingDelegate raw pointer in the reader is changing member to std::unique_ptr<UsdArnoldProcImagingDelegate>, using make_unique at construction.
• _volumes raw pointer vector in volume.h
std::vector<HdArnoldShape*> managed with scattered delete calls.
• reset(new X()) patterns in render_delegate.cpp are replaced with std::make_shared<HdResourceRegistry>()

6. Remove vestigial TF_UNUSED in openvdb_asset.

renderDelegate IS used

[Copilot]

Pull request overview

This PR targets correctness and hot-path performance improvements in libs/render_delegate/, primarily around render buffer thread-safety, array handling safety, and reducing avoidable O(n²)/allocation-heavy patterns.

Changes:

• Improves render buffer locking in HdArnoldRenderBuffer::Map/Unmap to avoid manual lock/unlock patterns.
• Replaces linear duplicate checks with hash-based sets for volume field and light-linking registration.
• Reduces transient string allocations in render settings by pre-reserving and appending.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
libs/render_delegate/volume.cpp	Switches volume-field duplicate detection to per-path hash sets and minor string handling cleanup.
libs/render_delegate/shared_arrays.h	Adds guards before accessing the first unboxed sample to avoid unsafe access on empty/unboxed data.
libs/render_delegate/render_settings.cpp	Reduces string reallocations when building derived node names (filter/user_data/aov shader names).
libs/render_delegate/render_delegate.h	Changes light-linking storage from vector to unordered_set to avoid O(n) duplicate checks.
libs/render_delegate/render_delegate.cpp	Updates resource registry/render param construction and light-linking registration to match new container choices; minor string handling optimization.
libs/render_delegate/render_buffer.cpp	Refactors Map/Unmap locking toward RAII and lock ownership transfer.
libs/render_delegate/openvdb_asset.cpp	Removes vestigial TF_UNUSED(renderDelegate) since the delegate is stored/used.
libs/render_delegate/instancer.cpp	Avoids repeated per-sample reserve() by reserving once at the maximum expected instance count.

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