Scott Lamb f9c46dca89 fix incorrect prev_media_duration_90k calculation
I spotted this by inspection: adding a media time and wall time didn't
look right. I also confirmed the brokenness on my primary NVR:

```
sqlite> .mode column
sqlite> select
   ...>   r1.composite_id,
   ...>   r1.prev_media_duration_90k,
   ...>   r1.wall_duration_90k,
   ...>   r1.media_duration_delta_90k,
   ...>   r2.composite_id,
   ...>   r2.prev_media_duration_90k
   ...> from
   ...>   recording r1 join recording r2 on (r1.composite_id = r2.composite_id - 1)
   ...> where
   ...>   r1.prev_media_duration_90k + r1.wall_duration_90k + r1.media_duration_delta_90k !=
   ...>     r2.prev_media_duration_90k
   ...> limit 5;
4296791095    2232623913716            5398956            154                       4296791096    2232629312672
4296791096    2232629312672            5400016            38                        4296791097    2232634712688
4296791097    2232634712688            5400729            105                       4296791098    2232640113417
4296791098    2232640113417            5399024            80                        4296791099    2232645512441
4296791099    2232645512441            5400770            124                       4296791100    2232650913211
```

In the first row, the second recording's prev_media_duration_90k is the
first's prev_media_duration_90k plus its wall time, not its media time.
2021-03-25 22:09:29 -07:00
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