Add visualisation groups to skins.

PiperOrigin-RevId: 458441645
Change-Id: I14324e8efda5142b2718a5cc9215228c7556dbf7

src/user/user_mesh.cc

@@ -1408,6 +1408,7 @@ mjCSkin::mjCSkin(mjCModel* _model) {
   rgba[0] = rgba[1] = rgba[2] = 0.5f;
   rgba[3] = 1.0f;
   inflate = 0;
+  group = 0;
 
   vert.clear();
   texcoord.clear();
@@ -1530,7 +1531,6 @@ void mjCSkin::Compile(const mjVFS* vfs) {
       throw mjCError(this, "unkown material '%s' in skin", material.c_str());
   }
 
-
   // set total vertex weights to 0
   vector<float> vw;
   size_t nvert = vert.size()/3;