Make benchmarks return size not save images

tests/ImageSharp.Benchmarks/Drawing/FillRectangle.cs

@@ -12,13 +12,14 @@ namespace ImageSharp.Benchmarks
     using CoreImage = ImageSharp.Image;
     using CoreRectangle = ImageSharp.Rectangle;
     using CoreColor = ImageSharp.Color;
+    using CoreSize = ImageSharp.Size;
     using System.IO;
     using System.Numerics;
 
     public class FillRectangle
     {
         [Benchmark(Baseline = true, Description = "System.Drawing Fill Rectangle")]
-        public void FillRectangleSystemDrawing()
+        public Size FillRectangleSystemDrawing()
         {
             using (Bitmap destination = new Bitmap(800, 800))
             {
@@ -30,29 +31,22 @@ namespace ImageSharp.Benchmarks
                     var pen = new Pen(Color.HotPink, 10);
                     graphics.FillRectangle(Brushes.HotPink, new Rectangle(10, 10, 190, 140));
                 }
-
-                using (MemoryStream ms = new MemoryStream())
-                {
-                    destination.Save(ms, System.Drawing.Imaging.ImageFormat.Bmp);
-                }
+                return destination.Size;
             }
         }
 
         [Benchmark(Description = "ImageSharp Fill Rectangle")]
-        public void FillRactangleCore()
+        public CoreSize FillRactangleCore()
         {
             CoreImage image = new CoreImage(800, 800);
 
             image.Fill(CoreColor.HotPink, new ImageSharp.Drawing.Shapes.RectangularPolygon(new CoreRectangle(10, 10, 190, 140)));
 
-            using (MemoryStream ms = new MemoryStream())
-            {
-                image.SaveAsBmp(ms);
-            }
+            return new CoreSize(image.Width, image.Height);
         }
 
         [Benchmark(Description = "ImageSharp Fill Rectangle - As Polygon")]
-        public void FillPolygonCore()
+        public CoreSize FillPolygonCore()
         {
             CoreImage image = new CoreImage(800, 800);
 
@@ -62,10 +56,7 @@ namespace ImageSharp.Benchmarks
                             new Vector2(200, 150),
                             new Vector2(10, 150) });
 
-            using (MemoryStream ms = new MemoryStream())
-            {
-                image.SaveAsBmp(ms);
-            }
+            return new CoreSize(image.Width, image.Height);
         }
     }
 }