x11: Open a X11 Display directly

Do the few remaining things that GDK is doing for us:
- Open and close the X11 Display
- Set up a GSource on the Display FD to handle events
- Allocate and free the content of XGenericEventCookie,
  to "unroll" the few XInput2 events that Mutter still
  does handle.

And remove the GdkDisplay we've so long relied on.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2864>

src/x11/meta-x11-event-source.c

@@ -0,0 +1,93 @@
+/*
+ * Copyright (C) 2023 Red Hat Inc.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
+ * 02111-1307, USA.
+ */
+
+#include "config.h"
+
+#include "meta-x11-event-source.h"
+
+typedef struct {
+  GSource base;
+  GPollFD event_poll_fd;
+  Display *xdisplay;
+} MetaX11EventSource;
+
+static gboolean
+meta_x11_event_source_prepare (GSource *source,
+                               int     *timeout)
+{
+  MetaX11EventSource *event_source = (MetaX11EventSource *) source;
+
+  *timeout = -1;
+
+  return XPending (event_source->xdisplay);
+}
+
+static gboolean
+meta_x11_event_source_check (GSource *source)
+{
+  MetaX11EventSource *event_source = (MetaX11EventSource *) source;
+
+  return XPending (event_source->xdisplay);
+}
+
+static gboolean
+meta_x11_event_source_dispatch (GSource     *source,
+                                GSourceFunc  callback,
+                                gpointer     user_data)
+{
+  MetaX11EventSource *event_source = (MetaX11EventSource *) source;
+  MetaX11EventFunc event_func = (MetaX11EventFunc) callback;
+  gboolean retval = G_SOURCE_CONTINUE;
+
+  while (retval == G_SOURCE_CONTINUE &&
+         XPending (event_source->xdisplay))
+    {
+      XEvent xevent;
+
+      XNextEvent (event_source->xdisplay, &xevent);
+      retval = event_func (&xevent, user_data);
+    }
+
+  return retval;
+}
+
+static GSourceFuncs meta_x11_event_source_funcs = {
+  meta_x11_event_source_prepare,
+  meta_x11_event_source_check,
+  meta_x11_event_source_dispatch,
+};
+
+GSource *
+meta_x11_event_source_new (Display *xdisplay)
+{
+  GSource *source;
+  MetaX11EventSource *event_source;
+
+  source = g_source_new (&meta_x11_event_source_funcs,
+                         sizeof (MetaX11EventSource));
+  g_source_set_name (source, "[mutter] MetaX11Display events");
+
+  event_source = (MetaX11EventSource *) source;
+  event_source->xdisplay = xdisplay;
+  event_source->event_poll_fd.fd = ConnectionNumber (xdisplay);
+  event_source->event_poll_fd.events = G_IO_IN;
+  g_source_add_poll (source, &event_source->event_poll_fd);
+
+  return source;
+}