Add mpeg4 support to showiframe (autodetect)

Use this ffmpeg command to produce such a file :

ffmpeg -y -i IMAGE.jpg -s SIZE -r 1 -t 00:00:01 -an \

	-vcodec libx264 -profile:v high -preset slower \

	-subq 12 -top 1 -flags +ildct -intra -q:v 12 \

	-x264opts "sar=1/1:tff=1" -f h264 OUTPUT.264

The sar option may need to be adjusted for images

on 720x576 widescreen (64/45), for HD 1/1 is correct

thanks to Erik Slagter