Fix mipmapLevel capitalization and a few minor mipmapLevel related cleanups

[acolwell]

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• I've followed the contributing guidelines to the best of my understanding
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• I've formatted my code according to Natron's code style
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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?

The bulk of this change is making the capitalization consistent for mipmap. There was a mix of mipMap and mipmap. Now everything except for a serialization field name uses the mipmap form.

I've also included a few other mipmap code related things that I came across while renaming.

• Replaced a few cases where mipmapLevels were declared as ints instead of unsigned ints.
• Removed a method that had an identical implementation as another similarly named method in ViewerInstance.
• Renamed OverlaySupport::getCurrentRenderScale() to getCurrentMipmapLevel() as discussed in a previous pull request.

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

Yes. I've verified that the code builds and all the unit tests still pass. There really aren't any major changes. Most of the int -> unsigned int changes happened around the proxy level selection code so I made sure that worked as expected on a few projects. Other than that there isn't really anything else to test other than making sure the code compiled.

[devernay]

thanks! LGTM!