[GcodePreview]Keep thumbnails transparency (#15575)

* Makes Gcode thumbnails transparent

* Don't dispose MemoryStream used in Bitmap

Follows up on the remarks shown here:
https://docs.microsoft.com/en-us/dotnet/api/system.drawing.bitmap.-ctor?view=dotnet-plat-ext-6.0#system-drawing-bitmap-ctor(system-io-stream)

src/modules/previewpane/GcodePreviewHandler/GcodePreviewHandlerControl.cs

@@ -106,10 +106,7 @@ namespace Microsoft.PowerToys.PreviewHandler.Gcode
             {
                 var bitmapBytes = Convert.FromBase64String(bitmapBase64);
 
-                using (var bitmapStream = new MemoryStream(bitmapBytes))
-                {
-                    thumbnail = new Bitmap(bitmapStream);
-                }
+                thumbnail = new Bitmap(new MemoryStream(bitmapBytes));
             }
 
             return thumbnail;