Fix QMutexLocker bug

[acolwell]

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• 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)

• 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?

Fixing GenericWatcher::scheduleBlockingTask() so that it actually holds the tasksMutex when it is pushing new task information.

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

Yes. I basically just did simple tests that make sure when the function gets called everything still seems to work. I did not notice any problems caused by this change.

Futher details of this pull request

I suspect it could fix potential race conditions, but I didn't ever actually encounter any issues like that. I just happened to come across this code and realized that the queue was not being protected like the original author probably intended.