Current code divides before increasing precision.

Also reduce upper bound for strength from 255 to 64.  This will prevent
an overflow in the SSSE3 and MMX filter_line code: delta is expressed as
an u16 being shifted by 2 to the left. If it overflows, having a
strength not above 64 will make sure that m is set to 0 (making the
m*m*delta >> 14 expression void).

A value above 64 should not make any sense unless gradfun is used as
a blur filter.

Signed-off-by: Anton Khirnov <anton@khirnov.net>