color: Emit signal when device is updated

Will be emitted when a device updated its color state. Will be used by
an X11 color manager implementation, and tests.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2165>

src/backends/meta-color-device.c

@@ -37,6 +37,7 @@ enum
 {
   READY,
   CHANGED,
+  UPDATED,
 
   N_SIGNALS
 };
@@ -322,6 +323,12 @@ meta_color_device_class_init (MetaColorDeviceClass *klass)
                   G_SIGNAL_RUN_LAST, 0,
                   NULL, NULL, NULL,
                   G_TYPE_NONE, 0);
+  signals[UPDATED] =
+    g_signal_new ("updated",
+                  G_TYPE_FROM_CLASS (klass),
+                  G_SIGNAL_RUN_LAST, 0,
+                  NULL, NULL, NULL,
+                  G_TYPE_NONE, 0);
 }
 
 static void
@@ -1133,4 +1140,6 @@ meta_color_device_update (MetaColorDevice *color_device,
                                                lut_size);
 
   meta_monitor_set_gamma_lut (monitor, lut);
+
+  g_signal_emit (color_device, signals[UPDATED], 0);
 }