xwayland: Add support for byte-swapped clients

Instructs Xwayland to allow/disallow connections from X11 clients with a
different endianess based on the "xwayland-allow-byte-swapped-clients"
setting.

This option has no effect if Xwayland does not support the command
option +byteswappedclients/-byteswappedclients.

Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/2576
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2785>

config.h.meson

@@ -106,3 +106,6 @@
 
 /* Whether the Xwayland -terminate supports a delay */
 #mesondefine HAVE_XWAYLAND_TERMINATE_DELAY
+
+/* Whether the Xwayland supports +/-byteswappedclients */
+#mesondefine HAVE_XWAYLAND_BYTE_SWAPPED_CLIENTS

meson.build

@@ -536,6 +536,7 @@ endif
 have_xwayland_initfd = false
 have_xwayland_listenfd = false
 have_xwayland_terminate_delay = false
+have_xwayland_byte_swapped_clients = false
 if have_xwayland
   xwayland_dep = dependency('xwayland', required: false)
 
@@ -597,6 +598,16 @@ if have_xwayland
   if (have_xwayland_terminate_delay)
     cdata.set('HAVE_XWAYLAND_TERMINATE_DELAY', 1)
   endif
+
+  # For Xwayland +/-byteswappedclients usage
+  if xwayland_dep.found()
+    have_xwayland_byte_swapped_clients = xwayland_dep.get_variable('have_byteswappedclients',
+                                                                   default_value: 'false') == 'true'
+  endif
+
+  if (have_xwayland_byte_swapped_clients)
+    cdata.set('HAVE_XWAYLAND_BYTE_SWAPPED_CLIENTS', 1)
+  endif
 endif
 
 optional_functions = [
@@ -678,6 +689,7 @@ summary('Profiler', have_profiler, section: 'Options')
 summary('Xwayland initfd', have_xwayland_initfd, section: 'Options')
 summary('Xwayland listenfd', have_xwayland_listenfd, section: 'Options')
 summary('Xwayland terminate delay', have_xwayland_terminate_delay, section: 'Options')
+summary('Xwayland byte-swapped clients', have_xwayland_byte_swapped_clients, section: 'Options')
 
 summary('Enabled', have_tests, section: 'Tests')
 summary('Core tests', have_core_tests, section: 'Tests')

src/wayland/meta-xwayland.c

@@ -903,6 +903,13 @@ meta_xwayland_start_xserver (MetaXWaylandManager *manager,
   args[i++] = "7";
 #endif
 
+#ifdef HAVE_XWAYLAND_BYTE_SWAPPED_CLIENTS
+  if (meta_settings_are_xwayland_byte_swapped_clients_allowed (settings))
+    args[i++] = "+byteswappedclients";
+  else
+    args[i++] = "-byteswappedclients";
+#endif
+
   if (meta_settings_is_experimental_feature_enabled (settings,
                                                      META_EXPERIMENTAL_FEATURE_AUTOCLOSE_XWAYLAND))
 #ifdef HAVE_XWAYLAND_TERMINATE_DELAY