Further tweak navmesh_generate_cheap_keepsession

More nav_generate_cheap_keepsession tweaks

More keepsession tweaks

Silly update to make sure all tweaks are live.

Dumb fix + progress prints

Completely reworked navmesh_generate_cheap_keepsession
Now goes through the entire map, looking for ungenerated seed poisitions, and then incrementally generates one seed position at a time.

Stupid fix

fixes/tweaks
let stair areas merge in specific circumstances
check for stair areas between merge loops
fix error

only place walkable marks when no navareas are already there

Tweaks!