AiWander: use navmeshes to validate pathgrid
Currently AiWander uses navmeshes only if there is no pathgrid.
If the pathgrid is present, but path is blocked for some reason (when the pathgrid is incorrect, for example), AI can get stuck.
As a possible solution, we can temporary make AiWander to use navmeshes if AI encountered an obstacle. If the destination is unreahible in the navmesh mode, AI should select an another destination.