qtdemux: Add parsing Sample Encryption box (senc)
Submitted by Seungha Yang
ISO/IEC 23001-7 Common Encryption defines Sample Encryption Box (senc).
The senc provides much simpler way to access Sample Auxiliary Information
than saio/saiz. Moreover, since senc box includes all the necessary
sample encryption information of corresponding movie fragment,
we can remove dependency between encrypted (and fragmented) streams.