Keyframe detection was inverted in cfe1a9d3 in order to fix keyframe
flags set for the sample attached to trac #7091. However, that sample is
errantly muxed.

As noted at
https://web.archive.org/web/20020803104640/http://www.pcisys.net:80/~melanson/codecs/film-format.txt,
the original keyframe detection logic is correct, and this patch
restores it.
Signed-off-by: James Almer <jamrial@gmail.com>
(cherry picked from commit 9f9f56e6)