uiList: fix a bug in computing visual active index (afaict, in case of reordering, was affecting "keep active visible" feature when resizing, and the "active label color" in custom themes).

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@61214 954f8c5b-7b00-dc11-b283-0030488c597c