Refine map generator spawn placement rules

Spawn generation generally tries to place spawns:

- In spacious areas
- Away from the center (true center or mirror lines)
- Away from a symmetry-projected spawn
- Away from previously placed spawns

This commit introduces the following adjustments:

- The spacing between sequentially placed spawns is relaxed.
- Fix spawn mines contributing too much to the space reservation.
- Factor rotations into anti-center biasing calculation.
- Preserve spacing information in center space fallback decisions.
- Use a linear (instead of binary) falloff for anti-center biasing.
- Enforce that spawns have a minimum buildable area around them.
- Enforce that symmetry-projected spawns have as much separation as
  sequentially placed spawns would.
- Allow spawns on or near roads.

OpenRA.Mods.Common/MapGenerator/Symmetry.cs

@@ -278,26 +278,5 @@ namespace OpenRA.Mods.Common.MapGenerator
 					action(projections, original);
 				}
 		}
-
-		/// <summary>
-		/// Returns true iff xy is within reservationRadius of the center of a given CellLayer. If
-		/// a mirroring is specified, the radius is measured from the mirror line instead of the
-		/// center point.
-		/// </summary>
-		public static bool IsCPosNearCenter<T>(
-			CPos cpos,
-			CellLayer<T> cellLayer,
-			int reservationRadius,
-			Mirror mirror)
-		{
-			CPos[] testPoints;
-			if (mirror == Mirror.None)
-				testPoints = RotateAndMirrorCPos(cpos, cellLayer, 2, Mirror.None);
-			else
-				testPoints = RotateAndMirrorCPos(cpos, cellLayer, 1, mirror);
-
-			var separation = (testPoints[1] - testPoints[0]).LengthSquared;
-			return separation <= reservationRadius * reservationRadius * 4;
-		}
 	}
 }