Adds the framework for loading and saving backgrounds with correct or incorrect angles. Backgrounds in FSO will all have fixed angles internally, and newly created backgrounds in FRED will have fixed angles, but existing mission files will have incorrect angles. The parser will determine whether the mission file contains fixed or unfixed angles and act accordingly. (Actually fixing the angles will be done in part 2.)
The Rotators_locked flag was created specifically as a lab option that didn't affect animations or intrinsic rotations, so return it to that scope. Also rename it to clarify that it applies to subsystems and to be future-proof for submodel translation.
This moves submodel rotation out of the AI pipeline and into object movement. With the exception of turret rotation (which remains in the AI code) this now allows all types of submodel rotation to happen in the same place. Many associated functions have also been cleaned up.
In addition to making the code cleaner and clearer, this will make it much easier for intrinsic rotations and new animations to be applied to non-ship objects, such as weapons.
Fabian Woltermann