Introduce IRenderable.IsDecoration and AsDecoration.

These are used to filter *Renderables that shouldn't
be copied for things like highlight or shadow.

OpenRA.Mods.RA/Render/RenderBuilding.cs

@@ -63,8 +63,10 @@ namespace OpenRA.Mods.RA.Render
 			foreach (var a in r)
 			{
 				yield return a;
-				if (disabled)
-					yield return a.WithPalette(wr.Palette("disabled")).WithZOffset(1);
+				if (disabled && !a.IsDecoration)
+					yield return a.WithPalette(wr.Palette("disabled"))
+						.WithZOffset(a.ZOffset + 1)
+						.AsDecoration();
 			}
 		}