vp8dec: reset loopfilter delta values at keyframes

while testing a VP8 encoder I found a mismatch between FFMPEG and libvpx
VP8 decoders. The reason for this mismatch is that FFMPEG doesn't reset
loopfilter delta values at keyframes. Patch that fixes the issue is below.
I've verified that the output of ffmpeg after this patch matches libvpx.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
