[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/UnitTests-GcodeThumbnailProvider/GcodeThumbnailProviderTests.cs

@@ -31,7 +31,7 @@ namespace GcodeThumbnailProviderUnitTests
             provider.GetThumbnail(256, out IntPtr bitmap, out WTS_ALPHATYPE alphaType);
 
             Assert.IsTrue(bitmap != IntPtr.Zero);
-            Assert.IsTrue(alphaType == WTS_ALPHATYPE.WTSAT_RGB);
+            Assert.IsTrue(alphaType == WTS_ALPHATYPE.WTSAT_ARGB);
         }
 
         [TestMethod]