Fixes parsing of subs box for IMSC 1 image subtitles after update of isoboxer library.
requested to merge github/fork/TobbeEdgeware/fix-image-subtitles-after-isoboxer-update into development
Created by: TobbeEdgeware
Fixes #1804 (closed).
Test content http://vm2.dashif.org/dash/vod/testpic_2s/imsc1_img.mpd now works again.