Fix a bug in preprocessing when filters=box only
Fix a bug in preprocessing when filters=box only

Spitfire1900/upscaler,nagadomi/waifu2x,zyhkz/waifu2x,zyhkz/waifu2x,zyhkz/waifu2x,zyhkz/waifu2x,nagadomi/waifu2x,nagadomi/waifu2x,nagadomi/waifu2x,Spitfire1900/upscaler,nagadomi/waifu2x