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/Renderer.cs

@@ -110,6 +110,8 @@ namespace OpenRA
 			{
 				Game.RunAfterTick(() =>
 				{
+					ChromeProvider.SetDPIScale(after);
+
 					foreach (var f in Fonts)
 						f.Value.SetScale(after);
 				});