cuepack: cues into passages that keep their timestamps

parity/cases/cuepack/backwards-times.ndjson

@@ -0,0 +1,3 @@
+{"start": 10.0, "end": 5.0, "text": "an end before its start"}
+{"start": 5.0, "end": 20.0, "text": "and one that overlaps it"}
+{"start": 0.0, "end": 1.0, "text": "and one before both"}