Certain files seem to lack the "default display window" data (well,
the flag that signals its presence/absence). Therefore a
standards-compliant parser would try to read that flag, but the data
read would belong to another flag (the "vui_timing_info_present_flag").
ffmpeg has a heuristic in place for detecting such invalid default
display window parameters. This commit implements the same heuristic
in mkvmerge.
Fixes#1907.
