runtime: add qa for fast_atan2f, removed a float equality test

The float equality check was probably never resulting in true, so I changed it to a threshold comparison. This also fixes the labelled FIXME. The table look up is at best .0015 rad off from a 0 angle, so 1.5e-3 threshold would probably work. I made it 1.5e-5 because there's some interpolation based on table lookup; to be honest I didn't go through and calculate what the threshold should be given resolution (probably ballpark of 7e-4). This will also speed up the cases where the angle is very close to 0.

I shall henceforth implement a better atan.