Fix T80970: Copy Rotation constraint incorrect under shear

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

Negate Shear in Copy Rotation

This patch negates the effects of shear in the Copy Rotation constraint, which causes incorrect results and even flipping on rare occasions.

Here is the bug report associated with the issue: T80970
This bug report has an example of the problem in a .blend file.

This patch will change existing .blend files if a copy rotation constraint is used with a target that has shearing -- instead of pointing in the "wrong" direction (due to the shear), the constrained object will copy the final rotation of the target, with the shearing applied.

Proposed Solution: Correct the matrix of the constraint target for shear.

Limitations of proposed solution:
- It's conceivable that someone has used the "incorrect behaviour" in a rig... although I doubt this. It may be necessary to create an "allow shear" checkbox that is disabled by default to maintain the legacy behaviour.

No UI changes are needed for this patch :)

Differential Revision: https://developer.blender.org/D8915

Constraints: Child-Of, set inverse matrix upon creation

Set the inverse matrix when the Child Of constraint is created. This
prevents the bone/object from jumping away when the constraint is added,
improving usability.

Reviewed by: sybren

Differential Revision: https://developer.blender.org/D8851