This was needed by the now-removed mov demuxer for QuickTime video, or
to be more specific, the Sorenson 3 video codec. QuickTime can
(probably) still decoded by libavcodec, but this field is not needed
for this.
The reference in demux_mkv was apparently for decoding QuickTime in
Matroska, using binary QuickTime codecs (QTX stuff). It's possible that
this has been broken with the binary codecs removal (see commit
aebfbbf2bd), because it removed related code from demux_mkv. On the
other hand, the code section in question was enabled only if binary
win32 codecs were enabled. The win32 codec loader worked on 32 bit x86
only. This means QuickTime-in-Matroska was broken on all other
architectures, including 64 bit x86. Despite being possibly broken on a
major platform, nobody has complained about it yet, and since I couldn't
find a sample of such a mkv file, so don't bother with it.
dd3e1217eb