Decrease opacity of taxels with zero value.

This prevents the taxels from being occluded by the convex hull for meshes that do not provide face connectivity.

PiperOrigin-RevId: 790420616
Change-Id: I543fd7b52e4625d921b6b5b5b77b4720020df42a

src/engine/engine_vis_visualize.c

@@ -910,6 +910,10 @@ void mjv_addGeoms(const mjModel* m, mjData* d, const mjvOption* vopt,
               }
             }
 
+            if (rgba[0]==0 && rgba[1]==0 && rgba[2]==0) {
+              rgba[3] = .1;
+            }
+
             // draw triangles, one per side
             START
             makeTriangle(thisgeom, pos[0], pos[1], pos[2], rgba);