Fixed a regression in processing small TimeSeekRange npt values

The limited operation support for the live simulator requires retrieving
 subranges from the index file. But in adapting the existing
 offsets_for_time_range() to serve both purposes, I disabled a subtle
 feature that always ensured we were getting at least one full gop -
 even when the range ended before the first I-frame index in the
 index file.

Re-enabling this subtle feature would make this function too complicated.
 So instead have defined offsets_covering_time_range() (to support
 TimeSeekRange npt lookups) and (tbd) offsets_within_time_range()
 for live sim support.

Tested with contrived index files. The old behavior appears to be
 restored.