Threshold dialog exhibits roundoff errs

[ctrueden]

The min and max values in the threshold dialog can be manipulated with the dark background button. When you do so the min/max cutoffs are succeptible to rounding issues due to calculating from bottom..cutoff and cutoff+1 .. top. The +1 here is done before scaling. The real effect is likely a nonissue but it looks bad. Perhaps when the image is of integral type the min/max values should always be rounded to integers.

Migrated-From: http://trac.imagej.net/ticket/1684