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

python/mujoco/structs.cc

@@ -1390,6 +1390,7 @@ PYBIND11_MODULE(_structs, m) {
       });
   DefineStructFunctions(mjVisualGlobal);
 #define X(var) mjVisualGlobal.def_readwrite(#var, &raw::MjVisualGlobal::var)
+  X(orthographic);
   X(fovy);
   X(ipd);
   X(azimuth);
@@ -2123,6 +2124,7 @@ This is useful for example when the MJB is not available as a file on disk.)"));
   X(distance);
   X(azimuth);
   X(elevation);
+  X(orthographic);
 #undef X
 
 #define X(var) DefinePyArray(mjvCamera, #var, &MjvCameraWrapper::var)
@@ -2153,6 +2155,7 @@ This is useful for example when the MJB is not available as a file on disk.)"));
   X(frustum_top);
   X(frustum_near);
   X(frustum_far);
+  X(orthographic);
 #undef X
 
 #define X(var) DefinePyArray(mjvGLCamera, #var, &MjvGLCameraWrapper::var)