remote: Fetch ScreenCast version when using Remote Desktop

The ScreenCast portal supports receiving session objects created by the
Remote Desktop portal. Encode that dependency by fetching both portals'
version when using Remote Desktop functions.

Closes flatpak#158
Fixes a1530a9

libportal/remote.c

@@ -424,7 +424,10 @@ get_remote_desktop_interface_version_returned (GObject *object,
   g_variant_get_child (ret, 0, "v", &version_variant);
   call->portal->remote_desktop_interface_version = g_variant_get_uint32 (version_variant);
 
-  create_session (call);
+  if (call->portal->screencast_interface_version == 0)
+    get_screencast_interface_version (call);
+  else
+    create_session (call);
 }
 
 static void