Add light radius attribute.

Also fix bug: light defaults were not being saved.

PiperOrigin-RevId: 625142278
Change-Id: I8ec3a5a06e9f314e98b7c34346db3c24b34d7e0b

src/user/user_api.h

@@ -347,6 +347,7 @@ typedef struct _mjsLight {         // light specification
   // frame
   double pos[3];                   // position
   double dir[3];                   // direction
+  double radius;                   // radius
   mjtCamLight mode;                // tracking mode
   mjString targetbody;             // target body for targeting
 

src/user/user_model.cc

@@ -1985,6 +1985,7 @@ void mjCModel::CopyTree(mjModel* m) {
       m->light_active[lid] = (mjtByte)pl->active;
       copyvec(m->light_pos+3*lid, pl->pos, 3);
       copyvec(m->light_dir+3*lid, pl->dir, 3);
+      m->light_radius[lid] = pl->radius;
       copyvec(m->light_attenuation+3*lid, pl->attenuation, 3);
       m->light_cutoff[lid] = pl->cutoff;
       m->light_exponent[lid] = pl->exponent;