Fix warnings and MSVC errors

[acolwell]

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• [x] I've followed the contributing guidelines to the best of my understanding
• [x] I've read and understood the contributing guidelines
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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?

This change fixes various issues that prevent the code from compiling with the MSVC compiler.

• Fixes an implicit cast from enum to bool in the HostOverlay code that I believe was an unintended bug.
• Removed a few declared but undefined functions.
• Fixed compiler errors related to windows.h by defining WIN32_LEAN_AND_MEAN and only included the actual header required instead of windows.h in some cases.
• Fixed a few minor preprocessor macro issues.
• Defined a << operator for nullptr_t in a test because it appears that MSVC doesn't define one.

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

Yes. The installer still builds and all the tests still pass. This change doesn't really change any behavior. It just makes slight adjustments to make the MSVC compiler happy.

Futher details of this pull request

This fixes MSVC compiling issues, but I do not have a full MSVC build working yet.

The HostOverlay fixes resolve the compiler errors I was encountering and preserves existing behavior (i.e. turns off "autokeying" when setting the value). It is not clear to me that this is correct since all the other setValue() calls near the ones that where fixed appear to pass in a KeyFrame object. I'm not familiar enough with this code to determine if this is correct or not so I just opted for preserving existing behavior for now. If we do decide to provide a KeyFrame object in the future, this should be easy to do in the new code.

[rodlie]

Looks ok, but note that I don't have anything setup to test Natron with MSVC. I do maintain a couple of applications that uses MSVC and Qt5, but I'm still on MSVC 2017 (15).

The HostOverlay fixes resolve the compiler errors I was encountering and preserves existing behavior (i.e. turns off "autokeying" when setting the value). It is not clear to me that this is correct since all the other setValue() calls near the ones that where fixed appear to pass in a KeyFrame object. I'm not familiar enough with this code to determine if this is correct or not so I just opted for preserving existing behavior for now. If we do decide to provide a KeyFrame object in the future, this should be easy to do in the new code.

I'm also not familiar with this code, so not much help here.