Rdef slider range 8758

See https://trac.openmicroscopy.org.uk/ome/ticket/8758

To test, open image in web image viewer, click 'Edit' channel rendering.
Note that the sliders only go to the 'min' / 'max' pixel intensity range.
Now try manually editing the values above the sliders. If you put in a value outside the min/max range, the slider will expand it's range accordingly.
When you hit Apply or Save, the range is validated to be within the Pixel range, E.g. 0 - 255 for int8 image.