Move computation of flex_vert0 to user_mesh.cc.

This removes the dependency of flex_vert0 from mj_flex and mjData.

PiperOrigin-RevId: 700382297
Change-Id: I0b6d023c2e1ba14fa92417b65bfc288f37c223ae

src/user/user_mesh.cc

@@ -2944,6 +2944,16 @@ void mjCFlex::Compile(const mjVFS* vfs) {
 
   // create bounding volume hierarchy
   CreateBVH();
+
+  // compute bounding box coordinates
+  vert0_.assign(3*nvert, 0);
+  const mjtNum* bvh = tree.Bvh().data();
+  for (int j=0; j < nvert; j++) {
+    for (int k=0; k < 3; k++) {
+      double size = 2*(bvh[k+3] - radius);
+      vert0_[3*j+k] = (vertxpos[3*j+k] - bvh[k]) / size + 0.5;
+    }
+  }
 }