Fixed problem with shaded selection sometimes not snapping to the edge of the slot when selecting a group of empty slots.  Bug caused by the fact that .offset() in some browsers (eg Firefox) returns decimal values calculated from styling rather than integer pixel values from rendering.

git-svn-id: http://svn.code.sf.net/p/mrbs/code/mrbs/trunk@2812 25f7e783-145e-4f2d-a9f8-dd7182bfe9c7