The earlier code had two problems:
1. For reference frames that are not directly output (happens unless
low_delay is set), the mb skip values referred to the next reference
frame to be decoded.
2. For non-reference frames, every macroblock was always considered
skipped.
This makes the output (worse than) useless; that no one ever
complained about this shows that this feature is not really used.
It is therefore removed.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>