Decode requested gain maps for SDR output

[gregbenz]

What changed

• Decode the gain map when the caller supplies a gain-map destination, including for SDR output.
• Route gain-map metadata failures through the existing HEIF/AVIF cleanup path.
• Add HEIF and AVIF regressions comparing gain maps returned by SDR and HDR decode paths.

Why

The public decoder supplies a gain-map output buffer and exposes it through uhdr_get_decoded_gainmap_image(). For SDR HEIF/AVIF output, decoding previously returned success without populating that requested buffer. Callers now receive the decoded gain map consistently, regardless of the requested primary-image transfer function.

Testing

• HEIF and AVIF regressions fail before the fix and pass afterward.
• Focused HEIF/AVIF tests: 2 passed.
• Full unit suite: passed.
• git diff --check

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