Flattening tracks sometimes splits contiguous clips into segments

### Incorrect Functionality

If a track has adjacent Gaps, then items below the boundary between those Gaps are sliced in an unexpected way.

## To Reproduce

Using this test file "bug_before_flatten_redacted.otio"...
[flatten_bug_repro.zip](https://github.com/AcademySoftwareFoundation/OpenTimelineIO/files/11872212/flatten_bug_repro.zip)

Run this command:
```
% otiotool -i bug_before_flatten_redacted.otio --flatten video -o out.otio
```

## Expected Behavior

Clips below adjacent Gaps should not be segmented by the flatten operation. In the provided example, `Clip #1` should be left as-is.

## Screenshots

Here is a screenshot before flattening (note the highlighted Gap on the top track)

Here is the result after flattening (note that `Clip #1` has been split in two)

Reproduce the issue with otiotool using bug_before_flatten_redacted.otio from flatten_bug_repro.zip and the documented --flatten video command. Trace the flatten operation from that CLI entry point and verify the fix by confirming that Clip #1 remains contiguous rather than being split at adjacent Gaps.

Written by the indexing model from the issue text.