avfilter/vf_showinfo: Fix erroneous results for mean and stdev with pixel bits >8

avfilter/vf_showinfo: Fix erroneous results for mean and stdev with pixel bits >8

Have tested with be and le pixel format on be and le system for >8bit.
System:
lmwang@ubuntu:~/ffmpeg.git.mips$ grep HAVE_BIGENDIAN config.h
ffmpeg.git git:(showinfo) ✗ grep HAVE_BIGENDIAN config.h

Test result:
1, yuv420p
./ffmpeg -f lavfi  -i color=black:duration=1:r=1:size=1280x720,format=yuv420p,showinfo
Master:
mean:[16 128 128] stdev:[0.0 0.0 0.0]
After applied the patch:
 mean:[16 128 128] stdev:[0.0 0.0 0.0]

2, yuv420p10le
./ffmpeg -f lavfi  -i color=black:duration=1:r=1:size=1280x720,format=yuv420p10le,showinfo
Master:
mean:[32 1 1] stdev:[32.0 1.0 1.0]
After applied the patch:
mean:[64 512 512] stdev:[0.0 0.0 0.0]

3, yuv420p10be
./ffmpeg -f lavfi  -i color=black:duration=1:r=1:size=1280x720,format=yuv420p10be,showinfo
Master:
mean:[32 1 1] stdev:[32.0 1.0 1.0]
After applied the patch:
mean:[64 512 512] stdev:[0.0 0.0 0.0]

Signed-off-by: Limin Wang <lance.lmwang@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>