When the input vector gets close to -Y, y and theta becomes totally
unreliable. It is thus necessary to compute the result in a different
way based on x and z. The code already had a special case, but:
- The threshold for using the special case was way too low.
- The special case was not precise enough to extend the threshold.
- The special case math had a sign error, resulting in a jump.
This adds tests for the computation precision and fixes the issues
by adjusting the threshold, and replacing the special case with one
based on a quadratic Taylor expansion of sqrt instead of linear.
Replacing the special case fixes the bug and results in a compatibility
break, requiring versioning for the roll of affected bones.
Differential Revision: https://developer.blender.org/D9551
16eafdadf6