Enable libjpeg-turbo jpeg7 feature to fix IMWRITE_JPEG_LUMA/CHROMA_QUALITY (fixes #1869)

[shimat]

Summary

• Adds the jpeg7 feature to the libjpeg-turbo vcpkg dependency so that JPEG_LIB_VERSION is 70 on Windows and manylinux-full builds
• Resolves the warning IMWRITE_JPEG_LUMA/CHROMA_QUALITY are not supported because JPEG_LIB_VERSION < 70
• ImwriteFlags.JpegLumaQuality and ImwriteFlags.JpegChromaQuality now work as expected on all platforms

Root cause

OpenCV's JPEG support comes from two different sources depending on platform:

Platform	JPEG source	JPEG_LIB_VERSION before	after
Windows x64, manylinux-full	vcpkg libjpeg-turbo	62 (6b compat default)	70
Linux Docker, ARM, manylinux-slim	OpenCV bundled libjpeg-turbo	70 (hardcoded in 3rdparty/libjpeg-turbo/CMakeLists.txt:137)	unchanged
WASM	WITH_JPEG=OFF	N/A	N/A

The vcpkg port for libjpeg-turbo already exposes a jpeg7 feature (-DWITH_JPEG7=ON). This PR opts in to that feature.

Notes

• The jpeg7 feature changes the libjpeg ABI (incompatible with libjpeg 6b). Since all libraries are statically linked here, this has no impact on end users.
• The vcpkg CI cache will be invalidated once and rebuilt automatically.
• No source code changes — only vcpkg.json.

Fixes #1869

[coderabbitai]

Caution

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix_jpeg_lib_version

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