Renamed Rect.CenterIn -> CenterRect.

The previous naming suggested that the `Rect` that the method was being
called on was the one being centered.

src/Avalonia.Controls/Image.cs

@@ -63,10 +63,10 @@ namespace Avalonia.Controls
                 Vector scale = Stretch.CalculateScaling(Bounds.Size, sourceSize);
                 Size scaledSize = sourceSize * scale;
                 Rect destRect = viewPort
-                    .CenterIn(new Rect(scaledSize))
+                    .CenterRect(new Rect(scaledSize))
                     .Intersect(viewPort);
                 Rect sourceRect = new Rect(sourceSize)
-                    .CenterIn(new Rect(destRect.Size / scale));
+                    .CenterRect(new Rect(destRect.Size / scale));
 
                 context.DrawImage(source, 1, sourceRect, destRect);
             }

src/Avalonia.Visuals/Rect.cs

@@ -238,7 +238,7 @@ namespace Avalonia
         /// </summary>
         /// <param name="rect">The rectangle to center.</param>
         /// <returns>The centered rectangle.</returns>
-        public Rect CenterIn(Rect rect)
+        public Rect CenterRect(Rect rect)
         {
             return new Rect(
                 _x + ((_width - rect._width) / 2),