Allow mods to customise the default rendering scale.

OpenRA.Game/WorldViewportSizes.cs

@@ -19,6 +19,7 @@ namespace OpenRA
 		public readonly int2 MediumWindowHeights = new int2(600, 900);
 		public readonly int2 FarWindowHeights = new int2(900, 1300);
 
+		public readonly float DefaultScale = 1.0f;
 		public readonly float MaxZoomScale = 2.0f;
 		public readonly int MaxZoomWindowHeight = 240;
 		public readonly bool AllowNativeZoom = true;