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/NullLoadScreen.cs

@@ -24,7 +24,7 @@ namespace OpenRA.Mods.RA
 				return;
 
 			// Draw a black screen
-			Game.Renderer.BeginFrame(float2.Zero, 1f);
+			Game.Renderer.BeginFrame(int2.Zero, 1f);
 			Game.Renderer.EndFrame( new NullInputHandler() );
 		}