Use slideshow/pict to gain better scale performance

50~60% cpu time (flomap->bitmap (flomap->scale (bitmap->flomap bm)))
8~16% cpu time (pict->bitmap (scale bm ratio))

Also use openssl/sha1 to calculate checksum to avoid unneed update

Implemented an CRC16 checksum algorithm, it slower than sha1,
cusume 20% cpu time.