cogl/scanout: Add 'scanout-failed' signal

This will later be emitted when a scanout failed, e.g. by the not-test commit
failing for some reason, or drmModePageFlip() failing even if the
pre-conditions for scanout in the simple KMS backend passed.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2854>

cogl/cogl/cogl-scanout.c

@@ -29,11 +29,27 @@
 
 #include "cogl-scanout.h"
 
+enum
+{
+  SCANOUT_FAILED,
+
+  N_SIGNALS
+};
+
+static guint signals[N_SIGNALS];
+
 G_DEFINE_INTERFACE (CoglScanout, cogl_scanout, G_TYPE_OBJECT)
 
 static void
 cogl_scanout_default_init (CoglScanoutInterface *iface)
 {
+  signals[SCANOUT_FAILED] =
+    g_signal_new ("scanout-failed",
+                  G_TYPE_FROM_INTERFACE (iface),
+                  G_SIGNAL_RUN_LAST,
+                  0, NULL, NULL, NULL,
+                  G_TYPE_NONE, 1,
+                  COGL_TYPE_ONSCREEN);
 }
 
 gboolean
@@ -54,3 +70,10 @@ cogl_scanout_blit_to_framebuffer (CoglScanout      *scanout,
   else
     return FALSE;
 }
+
+void
+cogl_scanout_notify_failed (CoglScanout  *scanout,
+                            CoglOnscreen *onscreen)
+{
+  g_signal_emit (scanout, signals[SCANOUT_FAILED], 0, onscreen);
+}

cogl/cogl/cogl-scanout.h

@@ -30,6 +30,7 @@
 
 #include "cogl/cogl-types.h"
 #include "cogl/cogl-framebuffer.h"
+#include "cogl/cogl-onscreen.h"
 
 #include <glib-object.h>
 
@@ -56,4 +57,8 @@ gboolean cogl_scanout_blit_to_framebuffer (CoglScanout      *scanout,
                                            int               y,
                                            GError          **error);
 
+COGL_EXPORT
+void cogl_scanout_notify_failed (CoglScanout  *scanout,
+                                 CoglOnscreen *onscreen);
+
 #endif /* COGL_SCANOUT_H */