Camera module should save background frames and levels/LUT with images

[campagnola]

Goal: when images are opened in MosaicEditor, they should look exactly as they looked onscreen when they were saved.

• Images should get userLevels and userLookupTable info keys. These may be modified by MosaicEditor like userTransform.
• Images should also get a backgroundImage info key that references a stored background frame, and a 'backgroundRemovalMode' key that can be either 'divide' or 'subtract'.
• Don't save duplicate background frames; multiple images may reference the same background file.