Paste X-Flipped Pose: Fix Mirrored Rotations For Keyframe Interpolation And Animation!

The current mirroring of poses does not take keyframes into account. It inverts certain values for the quaternion rotation which gives a perfectly mirrored STATIC pose, but once these values are interpolating to eachother they rotate in the shortest, but incorrect direction.

I think the pattern is positive positive negative negative, which technically mirrors the pose, but it works ass for animations. By manual tinkering i found the solution is that the pose should be mirrored as copy copy invert invert for keyframes that interpolate properly.

The solution is to copy over the values for W and X but invert Y and Z from the bone they're copied from. Really weird that this is an issue to begin with, considering how straight forward the solution seems.