add Math.round() to Pixel.correctLevel()

The old versions of setRed/Green/Blue cast doubles to ints whenever
necessary. I thought the new versions would, but they actually didn't
because they passed the values through Pixel.correctLevel(int). I
assumed Jython would downcast the doubles to ints, but that was wrong.

So, I added a Pixel.correctLevel(double) that rounds the value passed
in before passing it to Pixel.correctLevel(int).