qtdemux: Parsing sgpd/sbgp box to support per sample group encryption

Submitted by Seungha Yang

Link to original bug (#782224)

Description

ISO/IEC 23001-7 Common encryption specification defines seig box
which can override default protection parameters of Track Encryption box (tenc).
Note that, Sample to Group box (sbgp) and Sample Group Description box (sgpd)
are required for parsing seig box, since sbgp and sgpd are linked structure
and seig is one type of child box of sgpd.