change alf step size to 8.

[nuomi2021]

@QSXW current ALF step size is 4. could you help set ALF step size to 8? thank you

From Benjamin Bross:

for ALF where functions are in increments of 4 while 8 should be sufficient according to the spec.

[QSXW]

Yes sure.

[QSXW]

Fixed and closed this issue. Please check https://github.com/FFmpeg/FFmpeg/commit/442e94e5e476710dfbb9f611a86ac3c86156394c for more details.