#246 Visualization modes loose memory

[Lassi Paavolainen]

Opening huge datasets in different 3D visualization modes seem to end up with memory losses. Example 3D mode can take many hundred megabytes of memory while visualizing some 300MB dataset but almost nothing is freed when 3D mode is closed.

Sections mode: Similar bug as was in gallery panel. Dataset is saved as RGB in sections
panel that makes it take three times too much memory. This shouldn't be
done this way. Datasets should be in original type and only visualizations
should be RGB.