* This is experimental, the file format may change still!
* Helps reduce memory usage, keeps .blend files smaller, and makes
saving quicker when not editing multires.
* This is implemented at the customdata level, currently only the
multires displacements can be stored externally.
ToDo
* Better integration with object duplication/removal/..
* Memory is not yet freed when exiting sculpt mode.
* Loading only lower levels is not supported yet.
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