Entity fails to find path to center of clump of units.

[Freagarach]

A24 SVN 22574. In the attached replay, entity 670 (right in the second group of units) can't find its patch to the center of the clump of units and keeps walking. The behaviour starts around 1:10, turn 358. The behaviour is kind of reproducible, it happens often, though not always.

[Freagarach]

Replay.

[wraitii]

I was tempted to backlog this as generally unfixable, but in this particular instance I can actually fix it, I've realised I forgot to reject paths that are worse than the current path.

See ​Phab:D2135

[wraitii]

In 22609:

Unit Motion - Improve behaviour around obstructions and unreachable goals

Use the "pretend correct path" behaviour for short goals too. This is a fix for #5545, since the position check now works correctly without needing to add a tricky check for path-vs-path distance.

Simplify HandleObstructedMove to use ComputePathToGoal more. This means we occasionally compute a long path when stuck, which fixes two cases of stuck units reported in #5547.

Move some common calls into functions for convenience.

Make sure the short-path-waypoint-range-relaxing introduced in rP22526 doesn't happen for the last waypoint, which caused units to occasionally never get in range of the last waypoint.

Clear short path waypoints when computing a long path.

This won't fix all instances of unit dancing, but it should improve most of them.

Fixes #5545, Fixes #5547

Differential Revision: ​https://code.wildfiregames.com/D2135