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_io.c

@@ -152,6 +152,8 @@ void mj_defaultVisual(mjVisual* vis) {
   // global
   vis->global.fovy                = 45;
   vis->global.ipd                 = 0.068;
+  vis->global.azimuth             = 90;
+  vis->global.elevation           = -45;
   vis->global.linewidth           = 1.0;
   vis->global.glow                = 0.3;
   vis->global.offwidth            = 640;