GitLab

Summary:
`get_rotation_to_best_fit_xy` is useful to expose externally, however there was a bug (which we probably did not care about for our use case): it could return a rotation matrix with det(R) == −1.
The diff fixes that, and also makes centroid optional (it can be computed from points).

Reviewed By: bottler

Differential Revision: D39926791

fbshipit-source-id: 5120c7892815b829f3ddcc23e93d4a5ec0ca0013