Fix #2525 - map one ISD to one Cue, not one Cue per content element
Created by: davemevans
Ensures only ISDs with actual content are mapped to Cues, whilst only creating a single Cue per ISD. See #2525 (closed) for discussion.
I have tested with a number of the samples in the menu and also with some more complex (multiple active cues etc) content we have internally.
Please test and provide feedback ASAP so this has a chance to make it into 2.6.8.