MedianCut should scale all color values from [0, 248] -> [0, 255]

I did some tests and `MedianCut` tends to make images darker because all colors besides white are with their last 3 bits set to '0'.

> https://github.com/imagej/imagej1/blob/faaa7e0b29e91afff5cba11b8c0e700d9e976ea4/ij/process/MedianCut.java#L253-L254

The following replacement does a much better job:
```
// adjust values, i.e. we had the last 3 bits cut off: [0, 248] --> [0, 255]
r = (255 * r) / 248;
g = (255 * g) / 248;
b = (255 * b) / 248;
```
This also resets white to its original color.

Start in MedianCut.java at the linked lines 253-254 and inspect how the red, green, and blue values are adjusted after the last three bits are removed. Apply the reported scaling behavior and verify that the resulting colors cover the intended range and preserve white, addressing the darker-image symptom described in the issue.

Written by the indexing model from the issue text.