Fix visibility on webview2 when window was invisible previously

We set webview2 to be always visible for simplicity. This is more of a workaround. Microsoft is also working to fix it:
MicrosoftEdge/WebView2Feedback#1077

src/webview/win/mod.rs

@@ -180,6 +180,7 @@ impl WV for InnerWebView {
           }
         }
 
+        controller.put_is_visible(true);
         let _ = controller_clone.set(controller);
 
         if let Some(file_drop_handler) = file_drop_handler {