Check for duplicates in RiffVideo::readInfoListChunk (backport #9369)

[mergify]

We've had some security reports about RiffVideo::readInfoListChunk() because it's a bit slow when you give it a long list to process. The complexity of this code is linear but the constant factor cost of inserting an element into xmpData_ is high. I got a 4x speedup by copying the elements into a std::map first (to eliminate duplicate keys). But duplicate keys probably only happen in malformed input files, so I've also added an enforce to disallow duplicates.

I don't think this is a security issue because it's just slow performance. It's not a quadratic algorithm, just constant factor slowness.

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This is an automatic backport of pull request #9369 done by Mergify.

[mergify]

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