Rb 2.4 visual studio

[devernay]

Thanks for submitting a pull request! Please provide enough information so that others can review your pull request. Additionally, make sure you've done all of these things:

• [X] I've followed the contributing guidelines to the best of my understanding
• [X] I've read and understood the contributing guidelines
• [X] I've formatted my code according to Natron's code style
• [X] I've searched the pull requests tracker to ensure that this PR is not a duplicate

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?

Fix compilation with Visual Studio, and implement a more portable version of dirExists()

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

No, as I don't have Windows or VS