Add a 4x4 cdef_filter AVX2 implementation
Add a 4x4 cdef_filter AVX2 implementation

cdef_filter_4x4_8bpc_c: 2273.6
cdef_filter_4x4_8bpc_avx2: 113.6

Decoding time reduces to 15.51s for first 1000 frames of chimera 1080p,
from 23.1 before cdef_filter SIMD or 17.86 with only 8x8 cdef_filter
SIMD.

xiph/rav1e,xiph/rav1e