Add optional azimuth and elevation to initial free camera.

- Add `azimuth` and `elevation` to `visual.global`.
- Add `mjv_defaultFreeCamera`.
- Use `mjv_defaultFreeCamera` in `simulate`.

PiperOrigin-RevId: 465505244
Change-Id: I61e60f14a990d4e2d220958adb75c325001978ee

src/engine/engine_vis_init.h

@@ -43,6 +43,9 @@ MJAPI void mjv_freeScene(mjvScene* scn);
 // set default visualization options
 MJAPI void mjv_defaultOption(mjvOption* vopt);
 
+// set default free camera
+MJAPI void mjv_defaultFreeCamera(const mjModel* m, mjvCamera* cam);
+
 // set default camera
 MJAPI void mjv_defaultCamera(mjvCamera* cam);