xrandr: set stage to black before dpms off

redraws aren't processed when dpms has been turned
off, so we need to set the stage to black before
the screen goes dark, so that when it comes back
it's not showing stale contents.

src/backends/meta-stage.c

@@ -41,6 +41,7 @@ typedef struct {
 struct _MetaStagePrivate {
   MetaOverlay cursor_overlay;
   gboolean is_active;
+  gboolean is_black;
 };
 typedef struct _MetaStagePrivate MetaStagePrivate;
 
@@ -121,6 +122,9 @@ meta_stage_paint (ClutterActor *actor)
   MetaStage *stage = META_STAGE (actor);
   MetaStagePrivate *priv = meta_stage_get_instance_private (stage);
 
+  if (priv->is_black)
+    return;
+
   CLUTTER_ACTOR_CLASS (meta_stage_parent_class)->paint (actor);
 
   meta_overlay_paint (&priv->cursor_overlay);
@@ -261,3 +265,16 @@ meta_stage_set_active (MetaStage *stage,
    */
   clutter_stage_event (CLUTTER_STAGE (stage), &event);
 }
+
+void
+meta_stage_set_black (MetaStage *stage,
+                      gboolean   is_black)
+{
+  MetaStagePrivate *priv = meta_stage_get_instance_private (stage);
+
+  if (priv->is_black != is_black)
+    {
+      priv->is_black = is_black;
+      clutter_actor_queue_redraw (CLUTTER_ACTOR (stage));
+    }
+}

src/backends/meta-stage.h

@@ -57,6 +57,8 @@ void meta_stage_set_cursor (MetaStage     *stage,
 
 void meta_stage_set_active (MetaStage *stage,
                             gboolean   is_active);
+void meta_stage_set_black (MetaStage *stage,
+                           gboolean   is_black);
 G_END_DECLS
 
 #endif /* META_STAGE_H */

src/backends/x11/meta-monitor-manager-xrandr.c

@@ -43,6 +43,8 @@
 #include <meta/errors.h>
 #include "meta-monitor-config.h"
 
+#include "meta-stage.h"
+
 #define ALL_TRANSFORMS ((1 << (META_MONITOR_TRANSFORM_FLIPPED_270 + 1)) - 1)
 
 /* Look for DPI_FALLBACK in:
@@ -646,6 +648,23 @@ get_xmode_name (XRRModeInfo *xmode)
   return g_strdup_printf ("%dx%d", width, height);
 }
 
+static void
+update_stage_visibility_for_power_save_mode (MetaMonitorManager *manager)
+{
+  MetaBackend *backend = meta_get_backend ();
+  ClutterActor *stage = meta_backend_get_stage (backend);
+
+  if ((manager->power_save_mode != META_POWER_SAVE_ON) &&
+      (manager->power_save_mode != META_POWER_SAVE_UNSUPPORTED))
+    {
+      meta_stage_set_black (META_STAGE (stage), TRUE);
+    }
+  else
+    {
+      meta_stage_set_black (META_STAGE (stage), FALSE);
+    }
+}
+
 static void
 meta_monitor_manager_xrandr_read_current (MetaMonitorManager *manager)
 {
@@ -693,6 +712,8 @@ meta_monitor_manager_xrandr_read_current (MetaMonitorManager *manager)
       manager->power_save_mode = META_POWER_SAVE_UNSUPPORTED;
     }
 
+  update_stage_visibility_for_power_save_mode (manager);
+
   XRRGetScreenSizeRange (manager_xrandr->xdisplay, DefaultRootWindow (manager_xrandr->xdisplay),
 			 &min_width,
 			 &min_height,