[Avatars] Calculate 'sane' hue precissions

We used to get the numeric value of the entrire md5 string which is a
128bit integer. We would then devide this by the maxval of a 128bit int.

There is no need for such huge computations. As we just require a value
between 0 and 255. Thus using two 16 bit values is more than enough to
get the precision we need. By just taking the MSB we get nearly
identical results.