Create coordinate system conversion wrapper around Mirror

The Symmetry.Mirror enum was previously documented as defined in terms
of the WPos system, which happens to align with CPos for Rectangular
grid types. However, the existing code gets confused when attempting
to support RectangularIsometric, due to a lack of appropriate
conversion logic.

This change makes Mirror agnostic to specific coordinate systems and
introduces a WMirror wrapper that provides getters for the correct
Mirror configuration given the coordinate system being used.

OpenRA.Mods.Common/MapGenerator/NoiseUtils.cs

@@ -189,7 +189,7 @@ namespace OpenRA.Mods.Common.MapGenerator
 			MersenneTwister random,
 			CellLayer<int> cellLayer,
 			int rotations,
-			Symmetry.Mirror mirror,
+			Symmetry.WMirror wmirror,
 			int featureSize,
 			Func<int, int> ampFunc)
 		{
@@ -199,7 +199,7 @@ namespace OpenRA.Mods.Common.MapGenerator
 				random,
 				size,
 				rotations,
-				mirror,
+				wmirror.ForCPos(),
 				featureSize,
 				ampFunc);
 			CellLayerUtils.FromMatrix(cellLayer, noise);