These were creating dangerous interdependencies. It is better to do explicit conversions when needed.
As an added plus, this means that zstring.h no longer depends on name.h which was very annoying.
- made FlipLineSideRefs native, due to the SetLineSideRefs removal
- fixed a bug with FlipLineSideRefs that rendered upper and lower textures incorrectly due to incorrect sector references
- FlipLineSideRefs now should only work on single-sided lines
