[BUG] NanoVDB viewer fails to render with OpenCL/OpenGL backends

### Environment
**Operating System:** Windows 11
**Version / Commit SHA:** VDB 8.1.1, SHA: 4a186c2
**Other:** MSVC 19.29.30148.0, C++11

### Describe the bug
The NanoVDB viewer failed to render correctly with OpenCL or OpenGL selected as the backend (but CUDA worked)
- For fog volumes, an elongated cuboid was drawn.
- For level sets, only the background checkerboard pattern was drawn.
- For points, there's nothing but a blank screen.

### To Reproduce
1. Build NanoVDB with
```
cmake -DCMAKE_TOOLCHAIN_FILE=C:\vcpkg\scripts\buildsystems\vcpkg.cmake `
-DUSE_NANOVDB=ON -DOPENVDB_BUILD_CORE=OFF -DOPENVDB_BUILD_BINARIES=OFF -DOPENVDB_ROOT='C:\Program Files (x86)\OpenVDB' `
-DNANOVDB_BUILD_EXAMPLES=ON -DNANOVDB_USE_INTRINSICS=ON `
-DNANOVDB_USE_OPENVDB=ON -DNANOVDB_USE_TBB=ON -DNANOVDB_USE_GLFW=ON -DNANOVDB_USE_NATIVEFILEDIALOG=ON `
-DNANOVDB_USE_CUDA=ON -DNANOVDB_CUDA_KEEP_PTX=ON `
-DNANOVDB_BUILD_UNITTESTS=OFF -DNANOVDB_BUILD_BENCHMARK=OFF DNANOVDB_USE_MAGICAVOXEL=OFF ..
cmake --build . --parallel 4 --config Release --target install
```
2. Run the NanoVDB viewer and load a grid.
3. Switch the backend to OpenCL or OpenGL
4. See error

### Expected behavior
Correct rendering of the NanoVDB grids should appear.

Start by reproducing the issue with the supplied CMake configuration, Windows environment, and NanoVDB viewer, then compare rendering behavior across CUDA, OpenCL, and OpenGL backends. Done means fog volumes, level sets, and points render correctly when OpenCL or OpenGL is selected.

Written by the indexing model from the issue text.