[mutter/gbsneto/offscreen-fix: 3/4] clutter/offscreen-effect: Clear offscreen when pre-paint fails

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit 4afc5d0daa0e1bfa0b478eacf6ec476526e79802
Author: Georges Basile Stavracas Neto <georges stavracas gmail com>
Date:   Tue Jan 7 13:17:56 2020 -0300

    clutter/offscreen-effect: Clear offscreen when pre-paint fails
    
    Some ClutterOffscreenEffect subclasses, such as ClutterBrightnessContrastEffect,
    early-return FALSE in pre-paint before chaining up. It's an important optimization
    that avoids creating or updating the offscreen framebuffer.
    
    However, if an offscreen framebuffer already exists by the time pre-paint fails,
    it will be used *without* repaint the actor over it. That causes an old picture
    of the actor to be displayed.
    
    Fix that by always clearing the offscreen framebuffer when pre-paint fails.
    
    Fixes https://gitlab.gnome.org/GNOME/mutter/issues/810
    
    https://gitlab.gnome.org/GNOME/mutter/merge_requests/992

 clutter/clutter/clutter-offscreen-effect.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)
---
diff --git a/clutter/clutter/clutter-offscreen-effect.c b/clutter/clutter/clutter-offscreen-effect.c
index d83f0444a..89467e541 100644
--- a/clutter/clutter/clutter-offscreen-effect.c
+++ b/clutter/clutter/clutter-offscreen-effect.c
@@ -474,10 +474,17 @@ clutter_offscreen_effect_paint (ClutterEffect           *effect,
    */
   if (priv->offscreen == NULL || (flags & CLUTTER_EFFECT_PAINT_ACTOR_DIRTY))
     {
-      /* Chain up to the parent paint method which will call the pre and
-         post paint functions to update the image */
-      CLUTTER_EFFECT_CLASS (clutter_offscreen_effect_parent_class)->
-        paint (effect, paint_context, flags);
+      ClutterEffectClass *effect_class = CLUTTER_EFFECT_GET_CLASS (effect);
+      gboolean pre_paint_succeeded;
+
+      pre_paint_succeeded = effect_class->pre_paint (effect, paint_context);
+
+      clutter_actor_continue_paint (priv->actor, paint_context);
+
+      if (pre_paint_succeeded)
+        effect_class->post_paint (effect, paint_context);
+      else
+        g_clear_pointer (&priv->offscreen, cogl_object_unref);
     }
   else
     clutter_offscreen_effect_paint_texture (self, paint_context);