feat(viewer): don't use None and ScaleDown for object fit

src/viewer.rs

@@ -106,9 +106,10 @@ impl MMViewer {
         match self.fit {
             ObjectFit::Fill => self.fit = ObjectFit::Contain,
             ObjectFit::Contain => self.fit = ObjectFit::Cover,
-            ObjectFit::Cover => self.fit = ObjectFit::ScaleDown,
-            ObjectFit::ScaleDown => self.fit = ObjectFit::None,
-            ObjectFit::None => self.fit = ObjectFit::Fill,
+            ObjectFit::Cover => self.fit = ObjectFit::Fill,
+            // ObjectFit::ScaleDown => self.fit = ObjectFit::None,
+            // ObjectFit::None => self.fit = ObjectFit::Fill,
+            _ => self.fit = ObjectFit::Contain,
         }
         cx.notify();
     }