Fix for compilers that use C++17 by default.

[acolwell]

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• [x] I've followed the contributing guidelines to the best of my understanding
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PR Description

What type of PR is this? (Check one of the boxes below)

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Improvement (non-breaking change which does not add functionality nor fixes a bug but improves Natron in some way)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] My change requires a change to the documentation
  • [ ] I have updated the documentation accordingly

What does this pull request do?

Specifying the MathUtils namespace to resolve the ambiguity caused by the addition of std::clamp() in C++17.

This fixes a build error on Windows with a freshly installed MSYS2 where g++ uses c++17 by default.

Have you tested your changes (if applicable)? If so, how?

Yes. The code that uses MathUtils::clamp() properly builds after this change.