Copy texture coordinates from OBJ into flex. Fixes #2464.

PiperOrigin-RevId: 732109960
Change-Id: I506b3540ab5d489ce8f10d0c473919611ccb654a

src/user/user_flexcomp.h

@@ -106,6 +106,7 @@ class mjCFlexcomp {
   std::vector<bool> used;         // is point used by any element (false: skip)
   std::vector<int> element;       // flex elements
   std::vector<float> texcoord;    // vertex texture coordinates
+  std::vector<int> facetexcoord;  // face texture coordinates (OBJ only)
 
   // plugin support
   std::string plugin_name;