Fix TilingPath bugs affecting unofficial mods

TilingPath had some bugs when working with single-point segments,
which exist outside of the official mods. This change fixes a few
problems:

- Avoid unbounded length zero-cost, zero-progression loops that could
  be formed by an arbitrary number of single-point corners. These
  could pop up arbitrarily in the middle of paths.
- Avoid zero-progression loops short-circuiting looped paths.
- Fix missing end type matching during back-tracing stage.
- Enhance lint check to ensure starts/ends have a valid directions.
- Remove some dead code.

OpenRA.Mods.Common/Lint/CheckMultiBrushes.cs

@@ -37,6 +37,13 @@ namespace OpenRA.Mods.Common.Lint
 								foreach (var tile in multiBrush.PossibleTiles())
 									if (!templatedTerrainInfo.TryGetTerrainInfo(tile, out var _))
 										emitError($"Tileset {terrainInfoName} has invalid MultiBrush collection `{collectionName}`: tileset does not have tile {tile.Type},{tile.Index}");
+
+								if (multiBrush.Segment != null)
+								{
+									// Validate start and end have a direction.
+									_ = multiBrush.Segment.StartDirection;
+									_ = multiBrush.Segment.EndDirection;
+								}
 							}
 							catch (Exception e) when (e is ArgumentException || e is InvalidOperationException)
 							{