[Non-BE] Use previous subframe for starting external orientation interpolation. Fix frozen orientations.
Related to #570 (closed)
Fixes issues presented in #570 (closed). Use the last available subframe when setting the start for external orientation interpolation. Also fix frozen rotation values.
The non-BE tests should be related to external orientations only.
This MR fixes some stuff related to !770 (closed) and should be merged before that.