Blender Git Loki

fix atan2f input conditional

Suspicious conditional found by PVS-Studio T48917

Original code (from Blender?s initial open-source commit!) looks like
it?s testing inputs to atan2f, to avoid undefined function values.
Passing xn=0 does *not* always evaluate to 0 though? I?m not sure if
this is a coding error or was done for a desired visual result.

Also changed xn==0 to xn>=0 to avoid function call in more cases.

Good description and visualization of atan2f function:
http://en.cppreference.com/w/c/numeric/math/atan2