[DisplayServer] Add missing FEATURE_WINDOW_DRAG flag to Windows, X11 and Wayland display servers.

platform/linuxbsd/x11/display_server_x11.cpp

@@ -152,6 +152,7 @@ bool DisplayServerX11::has_feature(Feature p_feature) const {
 		case FEATURE_CLIPBOARD_PRIMARY:
 		case FEATURE_TEXT_TO_SPEECH:
 		case FEATURE_WINDOW_EMBEDDING:
+		case FEATURE_WINDOW_DRAG:
 			return true;
 		case FEATURE_SCREEN_CAPTURE:
 			return !xwayland;