Avoid redundantly setting viewport parameters in BeginFrame.

- Cache the old resolution, scroll and zoom in BeginFrame, and don't bother updating the viewport parameters again until they change.
- Pass around scroll as an int2 to reduce the number of back-and-forth casts.

OpenRA.Mods.RA/ModChooserLoadScreen.cs

@@ -38,7 +38,7 @@ namespace OpenRA.Mods.Cnc
 			if (r == null)
 				return;
 
-			r.BeginFrame(float2.Zero, 1f);
+			r.BeginFrame(int2.Zero, 1f);
 			WidgetUtils.FillRectWithSprite(bounds, sprite);
 			r.EndFrame(new NullInputHandler());
 		}