Keep stretch as the default resize mode in SkiaSharp resizer

Previously the SkiaSharp resizer ignored ImageResizeArgs.Mode entirely
and always stretched, so callers that never specified a mode were
implicitly relying on stretch output. Map None and Default to Stretch
to preserve that observable behavior while still honoring explicit
Max/Min/Crop/Pad/BoxPad values.

framework/src/Volo.Abp.Imaging.SkiaSharp/Volo/Abp/Imaging/SkiaSharpImageResizerContributor.cs

@@ -159,14 +159,11 @@ public class SkiaSharpImageResizerContributor : IImageResizerContributor, ITrans
             targetHeight = Math.Max(1, (int)Math.Round((double)source.Height * targetWidth / source.Width));
         }
 
-        var mode = resizeArgs.Mode == ImageResizeMode.Default ? ImageResizeMode.Crop : resizeArgs.Mode;
-        if (mode == ImageResizeMode.None)
-        {
-            mode = ImageResizeMode.Crop;
-        }
+        var mode = resizeArgs.Mode == ImageResizeMode.Default ? ImageResizeMode.Stretch : resizeArgs.Mode;
 
         switch (mode)
         {
+            case ImageResizeMode.None:
             case ImageResizeMode.Stretch:
                 return source.Resize(new SKImageInfo(targetWidth, targetHeight), Options.SKSamplingOptions);