Apparently some (win32) compilers are on the dumb side and don't know that
'unsigned int' and 'uint32_t' are the same thing. Thus calls to pvariance()
were having problems because the 3rd and 4th args are uint32_t but lum_mean
and lum_variance were 'unsigned int'. Changed lum_variance and lum_mean to
be uint32_t.
