Autocrop in 24-bit mode doesn't work very well

[DavidGriffith]

Pressing 'C' should automatically crop out surrounding whitespace. It works fine in 8-bit mode, but not 24-bit mode. A fix was presented in xv-20140327-mark-brader-24bit-autocrop-bug.msg which partially fixes the problem and the provided test image (a locomotive drawing) crops more or less correctly. I have another one of a old mobile phone that is cropped completely incorrectly by default, which is 24-bit mode. If I shift into 8-bit mode and crop there, the crop happens flawlessly.