Bug in mat4.Quaternion()

[johannes-graeter]

Hi,
thanks a lot for filling this gap in golang!
Lets get to it: I think there is a problem in mat4.Quaternion()
A. for calculating the trace only the part of the rotation matrix should be taken (Trace3) not the tracec of the full mat4
B. IMO you need to switch on the sign of the trace to account for ambiguities, right (see f.e. eigen quaternions line 822 following....)

Best
Johannes

[ungerik]

Thanks for the feedback. Don't have the capacity to work on this right now, so I would appreciate a PR