Update XML parsing to store asset path so that assets can be recovered and sent to external renderers

PiperOrigin-RevId: 567347908
Change-Id: I4d8e1abe35be3a67643ff925af709921280856f8

include/mujoco/mjmodel.h

@@ -593,6 +593,7 @@ struct mjModel_ {
   int nuser_sensor;               // number of mjtNums in sensor_user
   int nnames;                     // number of chars in all names
   int nnames_map;                 // number of slots in the names hash map
+  int npaths;                     // number of chars in all paths
 
   // sizes set after mjModel construction (only affect mjData)
   int nM;                         // number of non-zeros in sparse inertia matrix
@@ -777,6 +778,7 @@ struct mjModel_ {
   int*      mesh_graphadr;        // graph data address; -1: no graph         (nmesh x 1)
   mjtNum*   mesh_pos;             // translation applied to asset vertices    (nmesh x 3)
   mjtNum*   mesh_quat;            // rotation applied to asset vertices       (nmesh x 4)
+  int*      mesh_assetpathadr;    // address of asset path for mesh; -1: none (nmesh x 1)
   float*    mesh_vert;            // vertex positions for all meshes          (nmeshvert x 3)
   float*    mesh_normal;          // normals for all meshes                   (nmeshnormal x 3)
   float*    mesh_texcoord;        // vertex texcoords for all meshes          (nmeshtexcoord x 2)
@@ -983,6 +985,9 @@ struct mjModel_ {
   int*      name_pluginadr;       // plugin instance name pointers            (nplugin x 1)
   char*     names;                // names of all objects, 0-terminated       (nnames x 1)
   int*      names_map;            // internal hash map of names               (nnames_map x 1)
+
+  // paths
+  char*     paths;                // paths to assets, 0-terminated            (npaths x 1)
 };
 typedef struct mjModel_ mjModel;