Implement ConvertScalingMode properly (#596)

* Implement ConvertScalingMode properly

* Fix up the naming

* Only values 2 and 4 are allowed

* Return a nullable enum from ConvetScalingMode

* Fix typo on method name

* Use convertedScalingMode

Ryujinx.HLE/HOS/Services/Vi/ScalingMode.cs

@@ -1,24 +1,20 @@
-using System;
-using System.Collections.Generic;
-using System.Text;
-
-namespace Ryujinx.HLE.HOS.Services.Vi
+namespace Ryujinx.HLE.HOS.Services.Vi
 {
     enum SrcScalingMode
     {
-        Freeze = 0,
-        ScaleToWindow = 1,
-        ScaleAndCrop = 2,
-        None = 3,
+        None                = 0,
+        Freeze              = 1,
+        ScaleToWindow       = 2,
+        ScaleAndCrop        = 3,
         PreserveAspectRatio = 4
     }
 
     enum DstScalingMode
     {
-        None = 0,
-        Freeze = 1,
-        ScaleToWindow = 2,
-        ScaleAndCrop = 3,
+        Freeze              = 0,
+        ScaleToWindow       = 1,
+        ScaleAndCrop        = 2,
+        None                = 3,
         PreserveAspectRatio = 4
     }
 }