Huge pathfinding cost when unit stand on immobile terrain needs to move to attack

[dnqbob]

Testcase: use ABrandau/Shattered-Paradise-SDK@368813f
(engine version 0741439)

1. Choose map with cliffs
2. Use Mutant faction, build Queens
3. Make the queen spawn egg on cliff, wait for hatching
4. Build a fast unit (for example, Ravager) attracts the unit that stuck on cliff, then move out its attack range
5. Huge lag

[PunkPun]

the solution is to not spawn units on impassable terrain. The pathfinder was designed to handle this scenario in case of bugs, but it's not done efficiently

[dnqbob]