Correct cases where the X-axis of the bone (in pose space) aligns with
the pose-space Y or Z-axis. In these cases the decomposition of the
matrix fails, and a negative scale of the X-axis turns into a 180°
rotation around the Y-axis. An extra -1 scale to the X and Z axes of the
resulting matrix seems to fix things.
This adds a new RNA method `Action.flip_with_pose(ob)` to flip the
action channels that control a pose.
The rest-pose is used to properly flip the bones transformation.
This is useful as a way to flip actions used in pose libraries,
so the same action need not be included for each side.
Reviewed By: sybren
Ref D10781
