avfilter/vf_framerate: add SIMD functions for frame blending

avfilter/vf_framerate: add SIMD functions for frame blending

Blend function speedups on x86_64 Core i5 4460:

ffmpeg -f lavfi -i allyuv -vf framerate=60:threads=1 -f null none

C:     447548411 decicycles in Blend,    2048 runs,      0 skips
SSSE3: 130020087 decicycles in Blend,    2048 runs,      0 skips
AVX2:  128508221 decicycles in Blend,    2048 runs,      0 skips

ffmpeg -f lavfi -i allyuv -vf format=yuv420p12,framerate=60:threads=1 -f null none

C:     228932745 decicycles in Blend,    2048 runs,      0 skips
SSE4:  123357781 decicycles in Blend,    2048 runs,      0 skips
AVX2:  121215353 decicycles in Blend,    2048 runs,      0 skips

Signed-off-by: Marton Balint <cus@passwd.hu>