avfilter/vf_framerate: change blend factor precision

This is done mainly in preparation for the SIMD patches.

- for the 8-bit input, decrease the blend factor precision to 7-bit.
- for the 16-bit input, increase the blend factor precision to 15-bit.
- make sure the blend functions are not called with 0 or maximum blending
  factors, because we don't want the signed factor integers to overflow.

Fate test changes are due to different rounding.

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