Fixed bug ChannelSlider (rendering settings)

Fixes this bug: If the channel slider's  end value was already set to the slider's maximum, specifying a higher value in the text field didn't have any effect (same when specifying a lower value when the slider's start value was already set to the slider's minimum value).

Test: Choose an image with channels where pixels type maximum > channels type maximum respectively pixels type minimum < channels type minimum. Make sure the slider start and end values are set to the channels type min and max (== slider's min and max). Now try to specify start values and end values outside the range of the slider's min and max (but within the pixels type range). The preview should update accordingly (triggered by hitting return key, or text field looses focus). You should not be able to set values outside the pixels type range.