clutter/offscreen-effect: Add new create_pipeline() vfunc

The most annoying aspect of ClutterOffscreenEffect right now, and
the reason for all its subclasses to override pre-paint, is that
the pipeline creating isn't under subclasses' control. That means
all subclasses must ask ClutterOffscreenEffect to run pre-paint
and create the pipeline, then they all create their own pipelines
to paint.

To reduce this complexity, add a new create_pipeline() vfunc to
ClutterOffscreenEffect. Next commits will port effects to use this
vfunc when necessary.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1355

clutter/clutter/clutter-offscreen-effect.c

@@ -158,6 +158,25 @@ ensure_pipeline_filter_for_scale (ClutterOffscreenEffect *self,
                                    filter, filter);
 }
 
+static CoglPipeline *
+clutter_offscreen_effect_real_create_pipeline (ClutterOffscreenEffect *effect,
+                                               CoglTexture            *texture)
+{
+  ClutterOffscreenEffectPrivate *priv = effect->priv;
+  CoglContext *ctx =
+    clutter_backend_get_cogl_context (clutter_get_default_backend ());
+  CoglPipeline *pipeline;
+  float resource_scale;
+
+  resource_scale = clutter_actor_get_real_resource_scale (priv->actor);
+
+  pipeline = cogl_pipeline_new (ctx);
+  ensure_pipeline_filter_for_scale (effect, resource_scale);
+  cogl_pipeline_set_layer_texture (pipeline, 0, texture);
+
+  return pipeline;
+}
+
 static gboolean
 update_fbo (ClutterEffect *effect,
             int            target_width,
@@ -166,6 +185,8 @@ update_fbo (ClutterEffect *effect,
 {
   ClutterOffscreenEffect *self = CLUTTER_OFFSCREEN_EFFECT (effect);
   ClutterOffscreenEffectPrivate *priv = self->priv;
+  ClutterOffscreenEffectClass *offscreen_class =
+    CLUTTER_OFFSCREEN_EFFECT_GET_CLASS (effect);
 
   priv->stage = clutter_actor_get_stage (priv->actor);
   if (priv->stage == NULL)
@@ -185,15 +206,6 @@ update_fbo (ClutterEffect *effect,
     return TRUE;
   }
 
-  if (priv->pipeline == NULL)
-    {
-      CoglContext *ctx =
-        clutter_backend_get_cogl_context (clutter_get_default_backend ());
-
-      priv->pipeline = cogl_pipeline_new (ctx);
-      ensure_pipeline_filter_for_scale (self, resource_scale);
-    }
-
   g_clear_pointer (&priv->texture, cogl_object_unref);
   g_clear_pointer (&priv->offscreen, cogl_object_unref);
 
@@ -202,8 +214,6 @@ update_fbo (ClutterEffect *effect,
   if (priv->texture == NULL)
     return FALSE;
 
-  cogl_pipeline_set_layer_texture (priv->pipeline, 0, priv->texture);
-
   priv->target_width = target_width;
   priv->target_height = target_height;
 
@@ -212,8 +222,7 @@ update_fbo (ClutterEffect *effect,
     {
       g_warning ("%s: Unable to create an Offscreen buffer", G_STRLOC);
 
-      cogl_object_unref (priv->pipeline);
+      cogl_clear_object (&priv->pipeline);
-      priv->pipeline = NULL;
 
       priv->target_width = 0;
       priv->target_height = 0;
@@ -221,6 +230,9 @@ update_fbo (ClutterEffect *effect,
       return FALSE;
     }
 
+  cogl_clear_object (&priv->pipeline);
+  priv->pipeline = offscreen_class->create_pipeline (self, priv->texture);
+
   return TRUE;
 }
 
@@ -527,6 +539,7 @@ clutter_offscreen_effect_class_init (ClutterOffscreenEffectClass *klass)
   GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
 
   klass->create_texture = clutter_offscreen_effect_real_create_texture;
+  klass->create_pipeline = clutter_offscreen_effect_real_create_pipeline;
   klass->paint_target = clutter_offscreen_effect_real_paint_target;
 
   meta_class->set_actor = clutter_offscreen_effect_set_actor;

clutter/clutter/clutter-offscreen-effect.h

@@ -79,6 +79,8 @@ struct _ClutterOffscreenEffectClass
   CoglHandle (* create_texture) (ClutterOffscreenEffect *effect,
                                  gfloat                  width,
                                  gfloat                  height);
+  CoglPipeline* (* create_pipeline) (ClutterOffscreenEffect *effect,
+                                     CoglTexture            *texture);
   void       (* paint_target)   (ClutterOffscreenEffect *effect,
                                  ClutterPaintContext    *paint_context);