libvpxenc: add psnr support

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
libvpxenc: dont redundantly zero fields, the whole context is zeroed on init

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
ffmpeg: fix variable name in psnr printing code

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