* Split synth info initialzation by type.
We are going to remove the type enumeration and use a class hierarchy
instead. This preparation will then be helpful.
* Make Add...Parts() overrides of a virtual AddPart() function.
Since we have a class hierarchy for the different synthesized pieces, we
can now turn a case distinction into a virtual function call.
* Move initialization based on type to derived class constructors.
Move initialization of end transformations of flexible parts into
class lcSynthInfoCurved.
* Make GetDefaultControlPoints() virtual with overrides.
* Remove obsolete enum lcSynthType.
We have replaced its purpose by derived classes by now.
* Initialize shock absorbers' spring part ID early.
This removes the awkward early return that is needed in the if-else
cascade.
* Split lcSynthInfo into derived classes for curved and straight pieces.
* Only curved parts have varying sections, start, middle, and end properties.
Move the properties from the base class to the derived class that needs
them.
* Use derived classes to mark synthesized objects of different kinds.
We will extend the derived classes in the upcoming commits.
* PieceInfo is only needed to synthesize some hoses and shock absorbers.
* Initialize edge part IDs of flexible hoses early.
This removes another case distinction in AddParts().
* Verify the number of control points loaded from a model file.
* Synthesize Technic universal joints.
The direction of one end can be changed so that it points to the control
point.
* Technic universal joints need only the position of the control point.
* Synthesize legacy universal joints.
