Handle recoverable truncated image payloads

[rajantripathi]

Summary

• retry recoverable truncated PNG/JPEG payloads in the central base64 image decoder
• keep strict Pillow decoding as the default path
• limit Pillow's process-wide truncated-image flag to the recovery attempt and restore its previous value immediately
• add a regression test covering base64 TSV image decoding and flag restoration

Why

Some benchmark TSVs contain recoverable truncated image payloads. A single recoverable image should not cause an entire evaluation combination to fail, but permissive decoding should also not remain enabled globally.

Fixes #1400.

Validation

• rebased onto current upstream main
• python3 -m py_compile vlmeval/smp/vlm.py tests/test_smp_vlm.py: passed
• direct execution of the regression function: passed
• verified that the recovered image is loadable and retains its expected dimensions
• verified that ImageFile.LOAD_TRUNCATED_IMAGES is restored after recovery
• git diff --check origin/main...HEAD: passed

Full pytest collection is not available in the local minimal environment because pytest and the repository's optional benchmark dependency stack are not installed.

[rajantripathi]

Rebased this PR onto the current upstream main (e8e78f0) with no conflicts. The updated head is 178bf28.

Current validation:

• python3 -m py_compile vlmeval/smp/vlm.py tests/test_smp_vlm.py — passed
• Executed the regression test function directly — truncated-image recovery smoke test passed
• git diff --check origin/main...HEAD — passed
• PR remains mergeable and contains one commit over current main.

When convenient, could a maintainer review this fix for #1400? I am happy to adjust the implementation or test placement to match project preferences.

[rajantripathi]

Rebased onto the latest upstream main and tightened the implementation after another review. Strict Pillow decoding remains the default; only errors explicitly identified as truncated-image failures trigger a permissive retry, and the global Pillow flag is restored in a finally block. The regression test now verifies both successful recovery and flag restoration. Current head: a873252.