ISO/IEC 23001 spec section 7.2 specifies:

> InitializationVector SHALL conform to the definition specified in 9.2. Only one Per_ Sample_IV_Size SHALL be used within a file or Per_Sample_IV_Size SHALL be zero when a sample is unencrypted or a Constant IV is in use. Selection of InitializationVector values SHOULD follow the recommendations of 9.2.

The value should be inherited from the `tenc` box:

```
[moov] size=8+911
  [mvhd] size=12+96
...
                [schi] size=8+32
                  [tenc] size=12+20
                    default_isProtected = 1
                    default_Per_Sample_IV_Size = 8
```

But later in the dump you see that the code guesses (incorrectly) that the value is 0:

[moof] size=8+9781
  [mfhd] size=12+4
    sequence number = 17494866
  [traf] size=8+9757
...
        (       0) 3393
    [senc] size=12+6400, flags=2
      sample info count = 180
      IV Size (inferred) = 0
      sample info entries:
        (       0) 
          info = []
          sub entries:
            (       0) bytes_of_clear_data = 515, bytes_of_encrypted_data = 67438087
        (       1) 
```

This leads to bogus values for the sample entry table
mp4dump decodes `senc` box incorrectly for non-constant IV
This is a GitHub Issue
repo:Bento4
owner:axiomatic-systems
Title : mp4dump decodes `senc` box incorrectly for non-constant IV
Issue date:

--- start body ---
ISO/IEC 23001 spec section 7.2 specifies:

> InitializationVector SHALL conform to the definition specified in 9.2. Only one Per_ Sample_IV_Size SHALL be used within a file or Per_Sample_IV_Size SHALL be zero when a sample is unencrypted or a Constant IV is in use. Selection of InitializationVector values SHOULD follow the recommendations of 9.2.

The value should be inherited from the `tenc` box:

```
[moov] size=8+911
  [mvhd] size=12+96
...
                [schi] size=8+32
                  [tenc] size=12+20
                    default_isProtected = 1
                    default_Per_Sample_IV_Size = 8
```

But later in the dump you see that the code guesses (incorrectly) that the value is 0:

[moof] size=8+9781
  [mfhd] size=12+4
    sequence number = 17494866
  [traf] size=8+9757
...
        (       0) 3393
    [senc] size=12+6400, flags=2
      sample info count = 180
      IV Size (inferred) = 0
      sample info entries:
        (       0) 
          info = []
          sub entries:
            (       0) bytes_of_clear_data = 515, bytes_of_encrypted_data = 67438087
        (       1) 
```

This leads to bogus values for the sample entry table
--- end body ---
