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.HLE/UI/IHostUIHandler.cs

@@ -60,5 +60,10 @@ namespace Ryujinx.HLE.UI
         /// Gets fonts and colors used by the host.
         /// </summary>
         IHostUITheme HostUITheme { get; }
+
+        /// <summary>
+        /// Takes a screenshot from the current renderer and saves it in the screenshots folder.
+        /// </summary>
+        void TakeScreenshot();
     }
 }