Update libjpeg-turbo to 3.1.2 with SIMD enabled

[craigraw]

This PR is similar to (and borrows from) #86, but is different in two respects:

1. It updates libjpeg-turbo to the defined release of 3.1.2
2. It does not introduce any other build flags/logic

The current version of libjpeg-turbo in openpnp-capture is very old, and often MJPG capture fails with the message

ERROR tjDecompressHeader2 failed: Invalid handle

This newer version of the library appears to handle such cases.

[vonnieda]

Note: Closed and re-opened to re-run tests.

[vonnieda]

Thanks @craigraw, this is great!

Out of an abundance of caution, I downloaded the libjpeg-turbo source, verified it's signature and verified that none of the files had been changed per https://libjpeg-turbo.org/Downloads/DigitalSignatures

> curl -sSL 'https://raw.githubusercontent.com/libjpeg-turbo/repo/main/LJT-GPG-KEY' | gpg --import -
gpg: key 7EC2DBB6F4DBF434: public key "The libjpeg-turbo Project (Signing key for official binaries) <information@libjpeg-turbo.org>" imported
gpg: Total number processed: 1
gpg:               imported: 1

> gpg --verify libjpeg-turbo-3.1.2.tar.gz.sig 
gpg: assuming signed data in 'libjpeg-turbo-3.1.2.tar.gz'
gpg: Signature made Wed Sep  3 11:28:22 2025 CDT
gpg:                using RSA key 7EC2DBB6F4DBF434
gpg: Good signature from "The libjpeg-turbo Project (Signing key for official binaries) <information@libjpeg-turbo.org>" [unknown]
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: 0338 C8D8 D9FD A62C F9C4  21BD 7EC2 DBB6 F4DB F434

> mkdir tmp
> tar -xvzf libjpeg-turbo-3.1.2.tar.gz -C tmp
> diff -r libjpeg-turbo-3.1.2 tmp/libjpeg-turbo-3.1.2; echo $?
0