BUG: Update MultiVolumeExplorer - Fix frame scrolling

This fixes an issue due to missing cast from float to int that disabled
frame scrolling. See fedorov/MultiVolumeExplorer@f9222a25a

From: Andrey Fedorov <fedorov@bwh.harvard.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23521 3bd1e089-480b-0410-8dfb-8563597acbee