Fix logical errors

- field migrations require default values
- `clipped` referenced the wrong index for region, since it shifted
- missed an inclusion of `ratio` for detections in `process_frames`
- revert naming `o[2]` as `box` since it is out of scope!

This has now been test-run against a video, so I believe the kinks are
worked out.

Issue: #2948

migrations/007_add_object_filter_ratio.py

@@ -30,7 +30,9 @@ SQL = pw.SQL
 def migrate(migrator, database, fake=False, **kwargs):
     migrator.add_fields(
         Event,
-        ratio=pw.FloatField(),
+        ratio=pw.FloatField(
+            default=1.0
+        ),  # There is no way to get the true ratio from an existing recording, so simply assume they are square.
     )