Add a gdk_should_use_portal helper

We are going to use this in the following commits.

gdk/gdk-private.h

@@ -62,5 +62,6 @@ GDK_AVAILABLE_IN_ALL
 GdkPrivateVTable *      gdk__private__  (void);
 
 gboolean gdk_running_in_sandbox (void);
+gboolean gdk_should_use_portal (void);
 
 #endif /* __GDK__PRIVATE_H__ */

gdk/gdk.c

@@ -474,6 +474,26 @@ gdk_running_in_sandbox (void)
   return g_file_test ("/.flatpak-info", G_FILE_TEST_EXISTS);
 }
 
+gboolean
+gdk_should_use_portal (void)
+{
+  static const char *use_portal = NULL;
+
+  if (G_UNLIKELY (use_portal == NULL))
+    {
+      if (gdk_running_in_sandbox ())
+        use_portal = "1";
+      else
+        {
+          use_portal = g_getenv ("GTK_USE_PORTAL");
+          if (!use_portal)
+            use_portal = "";
+        }
+    }
+
+  return use_portal[0] == '1';
+}
+
 /**
  * gdk_display_open_default_libgtk_only:
  *