Use separate subpel filters for vertical and horizontal

[tdaede]

AKA dual-filter. Depends on #1726 .

[tdaede]

Bumping this for later as the compression gain is small.

[shssoichiro]

I'd be in favor of never implementing this and closing this issue (with a comment in the codebase as well). The speed to compression trade-off is so poor that dual filter is being removed in AV2.

[lu-zero]

I'm fine with it.