Do not cull mjGEOM_TRIANGLE.

PiperOrigin-RevId: 789723631
Change-Id: I8a5aaa78a9fe63458c07b823c7550846c14413b1

src/engine/engine_vis_visualize.c

@@ -911,12 +911,10 @@ void mjv_addGeoms(const mjModel* m, mjData* d, const mjvOption* vopt,
             }
 
             // draw triangles, one per side
-            for (int j = 0; j < 2; j++) {
-              START
-              makeTriangle(thisgeom, pos[0], pos[j ? 1 : 2], pos[j ? 2 : 1], rgba);
-              thisgeom->objid = id;
-              FINISH
-            }
+            START
+            makeTriangle(thisgeom, pos[0], pos[1], pos[2], rgba);
+            thisgeom->objid = id;
+            FINISH
           }
         }
       }

src/render/render_gl3.c

@@ -446,11 +446,15 @@ static void renderGeom(const mjvGeom* geom, int mode, const float* headpos,
     break;
 
   case mjGEOM_TRIANGLE:                       // triangle
+    glDisable(GL_CULL_FACE);
     glBegin(GL_TRIANGLES);
     glVertex3f(0, 0, 0);
     glVertex3f(size[0], 0, 0);
     glVertex3f(0, size[1], 0);
     glEnd();
+    if (scn->flags[mjRND_CULL_FACE]) {
+      glEnable(GL_CULL_FACE);
+    }
     break;
 
   case mjGEOM_FLEX:                           // flex