Bug fixes

- Rewrite documentation.
- Fix: Move the image in the preview in the direction it is dragged.
- Fix: Calculate a better relative exposure by minimizing the squared error.
- Fix: Build the image index, only modify pixels that are shown in the current image.
Fix false highlights.

- MetaData normalizes camera multipliers so that green is 1.
- Try to fix highlights in intermediate images so that they appear (mostly) gray after white balance.