qtdemux: fix offset calculation when parsing CENC aux info
Commit 7d7e54ce added support for DASH common encryption, however commit bb336840 that went onto master shortly before the CENC commit caused the calculation of the CENC aux info offset to be incorrect. The base_offset was being added if present, but if the base_offset is relative to the start of the moof, the offset was being added twice. The correct approach is to calculate the offset from the start of the moof and use that offset when parsing the CENC aux info.