SSSE3 implementations of film grain

gen_grain_y_ar0_8bpc_c: 84853.3
gen_grain_y_ar0_8bpc_ssse3: 23528.0
gen_grain_y_ar1_8bpc_c: 140775.5
gen_grain_y_ar1_8bpc_ssse3: 70410.2
gen_grain_y_ar2_8bpc_c: 251311.3
gen_grain_y_ar2_8bpc_ssse3: 95222.2
gen_grain_y_ar3_8bpc_c: 394763.0
gen_grain_y_ar3_8bpc_ssse3: 103541.9

gen_grain_uv_ar0_8bpc_420_c: 29773.7
gen_grain_uv_ar0_8bpc_420_ssse3: 7068.9
gen_grain_uv_ar1_8bpc_420_c: 46113.2
gen_grain_uv_ar1_8bpc_420_ssse3: 22148.1
gen_grain_uv_ar2_8bpc_420_c: 70061.4
gen_grain_uv_ar2_8bpc_420_ssse3: 25479.0
gen_grain_uv_ar3_8bpc_420_c: 113826.0
gen_grain_uv_ar3_8bpc_420_ssse3: 30004.9

fguv_32x32xn_8bpc_420_csfl0_c: 8148.9
fguv_32x32xn_8bpc_420_csfl0_ssse3: 1371.3
fguv_32x32xn_8bpc_420_csfl1_c: 6391.9
fguv_32x32xn_8bpc_420_csfl1_ssse3: 1034.8

fgy_32x32xn_8bpc_c: 14201.3
fgy_32x32xn_8bpc_ssse3: 3443.0