Add orthographic cameras.

Orthographic cameras are specified by setting the `orthographic` attribute of the `<camera>` element. The `fovy` attribute is still used to specify the field-of-view, but its semantic is different for orthographic cameras. For orthographic cameras, the field-of-view is expressed in units of length, rather than degrees.

Other related changes:

* Fix bug in the ordering of `cam_xxx` elements in `mjModel`.
* Make camera visualization translucent only when the frustum is visualized.
* Added a button to `simulate` to toggle between perspective and orthographic free cameras.

https://youtu.be/ZXBTEIDWHhs

PiperOrigin-RevId: 642293435
Change-Id: Id090a421ad88ab404b5b27ddbbd6bfc81ad49bc5

src/user/user_model.cc

@@ -2040,6 +2040,7 @@ void mjCModel::CopyTree(mjModel* m) {
       m->cam_targetbodyid[cid] = pc->targetbodyid;
       copyvec(m->cam_pos+3*cid, pc->pos, 3);
       copyvec(m->cam_quat+4*cid, pc->quat, 4);
+      m->cam_orthographic[cid] = pc->orthographic;
       m->cam_fovy[cid] = (mjtNum)pc->fovy;
       m->cam_ipd[cid] = (mjtNum)pc->ipd;
       copyvec(m->cam_resolution+2*cid, pc->resolution, 2);