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_objects.h

@@ -774,6 +774,8 @@ class mjCFlex: public mjCFlex_, private mjsFlex {
   void Compile(const mjVFS* vfs);         // compiler
   void CreateBVH(void);                   // create flex BVH
   void CreateShellPair(void);             // create shells and evpairs
+
+  std::vector<double> vert0_;             // vertex positions in [0, 1]^d in the bounding box
 };