Units can go through obstruction after long-path to the edge

[elexis]

As reported in #3410, units can go through obstructions. This ticket should deal with one particular case.

Reproduce:

1. Issue a walk order to the edge of a building or resource (The goal has to be far enough away so that the long-pathfinder will compute the path)
2. Once reached that point, issue a walk order to the center of the building.

If you enable the obstruction and pathfinder overlay in the developer menu you will be easier able to see if you selected a long-path and a waypoint that will trigger the error.

Using this recipe you can enter every static obstruction, buildings, resources, but not units and you can also not leave the map.

Notice a second bug: If you get a short-path instead of a long-one, you will not be able to get as close to the obstruction as you should be able to get. The unit will cancel the move earlier.

[elexis]

Replay with only two commands (cavalry inside CC, r17136)

[elexis]

Replay with only two commands (infantry inside CC, r17136)

[elexis]

Replay with the overlays enabled. The blue cell shows the goal of the longpath. Red are impassable cells. The bug is triggered reliably if the target (blue) is a neighboring cell of an impassable one (red).

[elexis]

Notice once the unit has finished the long-path, it is already standing inside the obstruction. The other command makes it only more visible.

[elexis]

The opaque blue square is the goal of the longpath. The blue outlined square is the obstruction of the cavalry. The blue line is the obstruction border of the civic centre. The teal / light blue line is the obstruction of the CC extended outwards by the size (clearance) of the cavalry (here 1). Notice the obstructions intersect significantly.

[wraitii]

In 17152:

Revert the logic change in r16998. This commit removed the checks in UnitMotion against structures, which should have been fine except the short-range pathfinder and the long-range pathfinder are not entirely compatible (check out #3532 for details). This behavior was probably slightly optimized, but it was too clever for its own good in the current state of the pathfinder, might be reintroduced later.

This resulted in ALL "units inside obstructions" issues.

Thanks to elexis for the testing.

Fixes #3532, #3450.
Refs #3538 (still OOSes), #3410 (unitmotion remains buggy for formations, but this is only aesthethic.)
Probably affects #3471 and #3505, but those are not fixed.