app: allow to toggle the histogram dialog between gamma and linear
Change GimpHistogram to take a "gboolean linear" parameter and always honor that parameter, so both kinds of histograms can now be created for all drawables. Add a horrible "Linear" toggle to the histogram dockable which always defaults to the active layer's actual pixel format, but can be switched at any time. This UI is ugly and needs to change. On the PDB, default to gamma-corrected if the plug-in is unaware of higher precision, and to the drawable's native pixel format otherwise. Other places using histograms (e.g. levels, curves) are unchanged.
Showing with 256 additions and 82 deletions