Add circle to flexcomp and remove rope and loop from composite.

Add pulley example to show a circle use case.

Switch on flex edge rendering by default (no effect when flex skin is rendered).

PiperOrigin-RevId: 700738379
Change-Id: I8469bbf194de01c8dab66c9a51c49879013df141

src/user/user_flexcomp.h

@@ -31,6 +31,7 @@ typedef enum _mjtFcompType {
   mjFCOMPTYPE_ELLIPSOID,
   mjFCOMPTYPE_SQUARE,
   mjFCOMPTYPE_DISC,
+  mjFCOMPTYPE_CIRCLE,
   mjFCOMPTYPE_MESH,
   mjFCOMPTYPE_GMSH,
   mjFCOMPTYPE_DIRECT,