Change the backdrop alpha in a screen orientation.

This change is related to issue #466 and PR #471. But the issue doesn't
reproduce again after this commit.

And then the backdrop alpha update should be necessary for the change of
the view size or its size classes.

See #572 for more detail.

Author: scenee

Sources/Core.swift

@@ -210,15 +210,12 @@ class Core: NSObject, UIGestureRecognizerDelegate {
         layoutAdapter.updateStaticConstraint()
         layoutAdapter.activateLayout(for: state, forceLayout: true)
 
-        // Update the backdrop alpha only when called in `Controller.show(animated:completion:)`
-        // Because that prevents a backdrop flicking just before presenting a panel(#466).
-        if forceLayout {
-            backdropView.alpha = getBackdropAlpha(for: state)
-        }
-
         if let contentOffset = contentOffset {
             scrollView?.contentOffset = contentOffset
         }
+
+        backdropView.alpha = getBackdropAlpha(for: state)
+
     }
 
     private func updateLayout(to target: FloatingPanelState) {