Orthogonalize the constraint target's matrix before decomposing it into
Euler angles. This removes sheer, and is actually a requirement for
correct decomposition.
It's conceivable that someone has used the incorrect behaviour in a rig.
As the shear caused unpredictable flipping of the constrained object,
this is unlikely.
Reviewed By: angavrilov, sybren
Differential Revision: https://developer.blender.org/D8915
849debe36c