aov.cpp, test_aov.py: fix shape_index for JIT variants

[LinasBeres]

Fix shape_index AOV for JIT/CUDA variants

Previously, the shape_index AOV reinterpreted the raw ShapePtr as a uint32, producing garbage values in JIT/CUDA variants instead of a proper scene index. This made the channel unusable for segmentation masks and per-shape identification in any JIT variant.

Further fixes #1059.

Description

The shape_index AOV channel is documented to return an integer index into the scene's shape list. In scalar variants this worked correctly. In JIT variants the raw ShapePtr value was cast to uint32, producing arbitrary large integers instead of indices in {0, 1, ..., N}.

Testing

Added test08_shape_index_correct in test_aov.py. The test renders a two-rectangle scene with an orthographic camera, box filter, and spp=1 to avoid fractional blending of indices.

Checklist

• My code follows the style guidelines of this project
• My changes generate no new warnings
• My code also compiles for cuda_* and llvm_* variants. If you can't test this, please leave below
• I have commented my code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I cleaned the commit history and removed any "Merge" commits
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