Add two new attributes to weld constraints:

- `anchor` determines the point of wrench application, in the frame of body2.
- `tfratio` scales applied torques relative to applied forces.
- Add visualisation of both anchor points to both weld and connect constraints.
- Add a test model showing how the new weld parameters behave.

PiperOrigin-RevId: 469190483
Change-Id: I20f6da85b09cd2c4c0b5d29eb8003eabb524e55e

src/user/user_objects.h

@@ -752,8 +752,8 @@ class mjCEquality : public mjCBase {
   double data[mjNEQDATA];         // type-dependent data
 
  private:
-  mjCEquality(mjCModel* = 0, mjCDef* = 0);// constructor
-  void Compile(void);                     // compiler
+  mjCEquality(mjCModel* = 0, mjCDef* = 0);  // constructor
+  void Compile(void);                       // compiler
 
   int obj1id;                     // id of object 1
   int obj2id;                     // id of object 2