Add fast approximations of x^2.4 and x^(1/2.4)

Use a chebyshev polynominal approximation of these to speed up
gamma conversion. Based on the post in:

http://stackoverflow.com/questions/6475373/optimizations-for-pow-with-const-non-integer-exponent/6478839#6478839

https://bugzilla.gnome.org/show_bug.cgi?id=678318