xwayland/window-manager: Add support for _NET_FRAME_EXTENTS

https://specifications.freedesktop.org/wm-spec/1.4/ar01s05.html says
"The Window Manager MUST set _NET_FRAME_EXTENTS to the extents of the
window's frame", so this is probably something we should be doing.

Some programs (such as some versions of Firefox) expect this to be present,
and will render popups in wrong locations if it's not.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>

shared/xcb-xwayland.c

@@ -78,6 +78,7 @@ x11_get_atoms(xcb_connection_t *connection, struct atom_x11 *atom)
 		{ "WM_S0",				F(wm_s0) },
 		{ "WM_CLIENT_MACHINE",			F(wm_client_machine) },
 		{ "WM_CHANGE_STATE",			F(wm_change_state) },
+		{ "_NET_FRAME_EXTENTS",			F(net_frame_extents) },
 		{ "_NET_WM_CM_S0",			F(net_wm_cm_s0) },
 		{ "_NET_WM_NAME",			F(net_wm_name) },
 		{ "_NET_WM_PID",			F(net_wm_pid) },

shared/xcb-xwayland.h

@@ -40,6 +40,7 @@ struct atom_x11 {
 	xcb_atom_t		 wm_s0;
 	xcb_atom_t		 wm_client_machine;
 	xcb_atom_t		 wm_change_state;
+	xcb_atom_t		 net_frame_extents;
 	xcb_atom_t		 net_wm_cm_s0;
 	xcb_atom_t		 net_wm_name;
 	xcb_atom_t		 net_wm_pid;