Editor: do not resize button if "new" Image value is the same as old

This prevents buttons resizing to the image's size in case user calls Sprite Selector dialog and cancels it.
This helps if button was previously resized manually after assigning a image.

Editor/AGS.Editor/Panes/GUIEditor.cs

@@ -182,7 +182,7 @@ protected override void OnPropertyChanged(string propertyName, object oldValue)
 				if ((_selectedControl != null) && (_selectedControl is GUIButton))
 				{
 					GUIButton selectedButton = (GUIButton)_selectedControl;
-					if (selectedButton.Image > 0)
+					if (selectedButton.Image > 0 && (selectedButton.Image != (int)oldValue))
 					{
                         int newWidth, newHeight;
                         Utilities.GetSizeSpriteWillBeRenderedInGame(selectedButton.Image, out newWidth, out newHeight);