Security Issue: dlib depends on vulnerable libpng version (CVE-2019-17371)

[cx418y]

Title: Security Issue: dlib depends on vulnerable libpng version (CVE-2019-17371)

Body:

Hi,

First of all, thank you for your work on dlib — it's a great library!

While analyzing third-party dependencies used in dlib v19.24.8（the newest version）, I found that it depends on libpng version 1.6.37 ([source](https://github.com/pnggroup/libpng/tree/v1.6.37)), which has a known security vulnerability:

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CVE-2019-17371

• Description: Memory leak in the functions png_malloc_warn (in [pngmem.c](https://github.com/davisking/dlib/blob/v19.24.8/dlib/external/libpng/pngmem.c)) and png_create_info_struct (in [png.c](https://github.com/davisking/dlib/blob/v19.24.8/dlib/external/libpng/png.c)) .
• Affected version: 1.6.37
• More info:
  • [NVD Entry](https://nvd.nist.gov/vuln/detail/CVE-2019-17371)
  • memory leak in png_malloc_warn and png_create_info_struct pnggroup/libpng#307
  • https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-17371

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This vulnerability may lead to memory leaks or potentially be exploited in specific contexts. Although the impact may be limited in some use cases, it's generally recommended to update to a patched version to minimize security risks.

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Suggested Fix:

Please consider updating the bundled libpng version in dlib to a newer, non-vulnerable release (e.g., 1.6.38 or later), or document the potential risk if updating is not currently feasible.

Let me know if you'd like help testing with a newer version, or if more information is needed.

Thanks again!