Add support for senc and IV size

[zdanek]

Hi.
I've added support for senc box.

I suppose that there was no support for this box as it depends on IV size that is specified in tenc box that sometimes is not present. For example for segments without Init segment. I've added support for saving IV size set in tenc if it's present.

If IV size is not present in preceding tenc box, reading senc is based on heuristics. It's size can be 0 (cbcs encoding) 8 or 16 bytes. It can guess that it should be 0 bytes by fitting encryption data size * sample_count against provided box size (len).
It works as supposed. I've checked it with different segments.

I'm open for suggestions.

[zdanek]

BTW temporarily I've put my version of this code at https://mlynoteka.mlyn.org/mp4parser/
I've fixed Samples for test option as original that refers dashif.org simply does not work!