ROI -AND- operation when holding down shift and alt

[aschain]

When drawing ROIs, shift will perform an OR operation on previous ROIs, and alt (option) will perform a NOT operation. Here, I have added an AND operation when the user holds shift and alt together. This will only keep the part of the previous ROI that is within the new ROI.

This has been useful to me, for example, when I have an accidentally large wand selection, and I only want to keep a small part of the selection.

[imagejan]

@rasband what do you think about this suggestion?