Bug 750594 - Colors in PSD files are far lighter on import and far darker on export

GIMP's PSD plugin was assuming that colors in PSD files are linearly interpolated
on both export and import. This commit fixes the issue by doing the following:

- when importing, assume the colors are gamma interpolated in the entire image
- when importing and exporting, treat the colors in each layer as gamma interpolated
- when importing and exporting, treat indexed colors as actually indexed colors
- when exporting, treat the interpolation in the channels and masks as linearly interpolated