Fix more QMutexLocker usage.

[acolwell]

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PR Description

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

• 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?

Fixes more instances of unnamed QMutexLocker objects that were not holding the lock for the duration of the containing scope.

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

Yes. Built locally and tested with by doing a bunch of zoom and userRoi changes. I didn't notice any issues like deadlocking or odd behavior. This is a pretty low risk fix since all the logic covered by the locks are simple member variable updates.

[devernay]

LGTM!
Thank you!