Optimize atan2 function

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atan2 needs optimizing so that it maintains accuracy over the full range of 
fixed point numbers while being as fast as Joes implementation (see wiki).

It may be that once tested Joes algorithm is as accurate across the full range, 
if this is the case then we can use this implementation.

Original issue reported on code.google.com by Flatmush@googlemail.com on 24 Feb 2011 at 1:47