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/xml/xml_native_reader.cc

@@ -794,6 +794,7 @@ const mjMap fcomp_map[mjNFCOMPTYPES] = {
   {"ellipsoid",   mjFCOMPTYPE_ELLIPSOID},
   {"square",      mjFCOMPTYPE_SQUARE},
   {"disc",        mjFCOMPTYPE_DISC},
+  {"circle",      mjFCOMPTYPE_CIRCLE},
   {"mesh",        mjFCOMPTYPE_MESH},
   {"gmsh",        mjFCOMPTYPE_GMSH},
   {"direct",      mjFCOMPTYPE_DIRECT}