we found some very old videos which suffered from
corruption after 9e6a2427, but were fine
before.
These had "End of AC stream reached in vp6_parse_coeff" warnings in logs.
These also had flv Packet mismatch warnings.
Adding FlixEngine to the list of flv muxers which produce broken packet
sizes fixes this corruption.

FlixEngine is very old and not maintained or available anymore (since
2010), so we won't need to worry about newer versions fixing the issue.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>