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

include/mujoco/mjvisualize.h

@@ -192,6 +192,9 @@ struct mjvCamera_ {               // abstract camera
   mjtNum   distance;              // distance to lookat point or tracked body
   mjtNum   azimuth;               // camera azimuth (deg)
   mjtNum   elevation;             // camera elevation (deg)
+
+  // orthographic / perspective
+  int      orthographic;          // 0: perspective; 1: orthographic
 };
 typedef struct mjvCamera_ mjvCamera;
 
@@ -211,6 +214,9 @@ struct mjvGLCamera_ {             // OpenGL camera
   float    frustum_top;           // top
   float    frustum_near;          // near
   float    frustum_far;           // far
+
+  // orthographic / perspective
+  int      orthographic;          // 0: perspective; 1: orthographic
 };
 typedef struct mjvGLCamera_ mjvGLCamera;
 
@@ -487,10 +493,12 @@ struct mjvSceneState_ {
     mjtNum* site_size;
     float* site_rgba;
 
+    int* cam_orthographic;
     mjtNum* cam_fovy;
     mjtNum* cam_ipd;
-    float* cam_intrinsic;
+    int* cam_resolution;
     float* cam_sensorsize;
+    float* cam_intrinsic;
 
     mjtByte* light_directional;
     mjtByte* light_castshadow;