Prevent compiler warnings in select0r

[rrrapha]

This stops compiler (clang) warnings like the following:

warning: absolute value function 'fabsf' given an argument of type 'double' but has parameter of type 'float' which may cause truncation of value [-Wabsolute-value]
ax = 0.5 - fabsf(fabsf(hue - chue) - 0.5);