Fixed bug where ghost for annotations being dragged would sometimes disappear behind other tracks, when other tracks have a background color set (discovered when working on MAKER dark background styles).
Still need to clean up: 
	base zIndex on retrieving actual zIndex from CSS rather than hardwiring
	check on render relative to sequence track -- could make ghost disappear if over seq track.  But I think rendering over seq track may actually be desirable behavior to essentially get DNA overlay for an evidence element.