CompareImages: Fix the A-B visualization mode

[vallsv]

The previous implementation was trying to constraint the data on the same data range, anyway the visualization mode.

This approach is fine for most of the modes, except A-B.

• Now A-B is handled in a special way, in order to use the basic operation A-B
• The colormap is now updated (when we switch between the visualization mode) in order to stay in the same data range
• Plus an RGB improvement to use any colormap normaization (before it was only a linear scale)

No changelog is needed as it's just a kind of partial revert of the previous change

Changelog:

[t20100]

LGTM
Good to have set|getVisualizationMode on the item