Add configurable octree max depth for meshes.

The maximum depth of the octree used for SDF generation can now be specified in the mesh definition via `mjsMesh::octree_maxdepth`. The default value is 6.

PiperOrigin-RevId: 912494999
Change-Id: I6d828d1d3999b99d1210a6829a05b04fac591e1f

src/user/user_init.c

@@ -243,6 +243,7 @@ void mjs_defaultMesh(mjsMesh* mesh) {
   mesh->scale[0] = mesh->scale[1] = mesh->scale[2] = 1;
   mesh->maxhullvert = -1;
   mesh->inertia = mjMESH_INERTIA_LEGACY;
+  mesh->octree_maxdepth = 6;
 }