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/engine/engine_vis_init.c

@@ -93,7 +93,7 @@ const char* mjVISSTRING[mjNVISFLAG][3] = {
   {"Static Body",     "1", "D"},
   {"Skin",            "1", ";"},
   {"Flex Vert",       "0", ""},
-  {"Flex Edge",       "0", ""},
+  {"Flex Edge",       "1", ""},
   {"Flex Face",       "0", ""},
   {"Flex Skin",       "1", ""},
   {"Body Tree",       "0", "`"},