remove pointless logic

Calculating the total autoWidth separately serves no purpose.
Subtracting it all at once or iteratively has the same result.
fix remainder width not applied to auto width columns

The while loop to distribute extra pixels had a condition that
prevented it from ever running.

I also added a unit test for this logic.
Merge pull request #1983 from Envisage/master

Yet another fix for auto-width columns + unit test