Support rendering at non-integer display scales:

* 2x and 3x DPI artwork can be specified using
  Image2x and Image3x in chrome.yaml.
* Images are rendered using bilinear interpolation.
* For non-integer screen scales, prefer downscaling
  the next biggest resolution image over upscaling.

OpenRA.Game/Graphics/Sheet.cs

@@ -25,6 +25,7 @@ namespace OpenRA.Graphics
 
 		public readonly Size Size;
 		public readonly SheetType Type;
+		public float DPIScale = 1f;
 
 		public byte[] GetData()
 		{