Use Lanczos2Lut for the native resize test

This should make the native method a bit faster
Avoid copying the bytes when encoding

Since we're calling free on the pointer after the data has been
written to the io.Writer, we can use the mem pointer as the backing
of the []byte without any copying.
Add a few more tests for performance vs std library
Add script for comparing benchmark results
Move quantum related macros to quantum.h
Add support for colorspaces
Make sure the stats were returned before trying to process them
Fix crash when computing statistics with IM backend

GetImageChannelStatistics returns ChannelStatistics *, not
ChannelStatistics **.
Always normalize statistics values to [0, 1]
Add function for computing image average color