Fix SaveCurrentScreenshot

This fixes SaveCurrentScreenshot so it correctly saves the screenshot into the screenshot folder, it's no longer a stub

I was going to add the capture button so all games worked but tbh I only care for spongebob

games tested
Pokemon Z-A: https://files.raychu.xyz/u/1FaUGV.png
Pokemon Violet: https://files.raychu.xyz/u/6swfVS.png
Pokemon Violet: https://files.raychu.xyz/u/JaBBX2.png
Spongebob The Cosmic Shake: https://files.raychu.xyz/u/8z5X2e.png

src/Ryujinx/Headless/WindowBase.cs

@@ -571,5 +571,10 @@ namespace Ryujinx.Headless
                 SDL2Driver.Instance.Dispose();
             }
         }
+
+        public void TakeScreenshot()
+        {
+            throw new NotImplementedException();
+        }
     }
 }