Blender Git Loki

Fix T83196: bad matrix to quaternion precision near 180 degrees rotation.

Adjust the threshold for switching from the base case to trace > 0,
based on very similar example code from www.euclideanspace.com to
avoid float precision issues when trace is close to -1.

Also, remove conversions to and from double, because using double
here doesn't really have benefit, especially with the new threshold.

Finally, add quaternion-matrix-quaternion round trip tests with
full coverage for all 4 branches.

Differential Revision: https://developer.blender.org/D9675