clutter/paint-nodes: Add new ClutterLayerNode API

ClutterLayerNode is the "offscreen framebuffer" node, that paints it's
child nodes in a separate framebuffer, and then copies that framebuffer
to the parent one.

It'll be useful to hand ClutterLayerNode which framebuffer and pipeline
to use, as this is a requirement for porting e.g. ClutterOffscreenEffect
and subclasses.

Add a new clutter_layer_node_new_to_framebuffer() API.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1340>

clutter/clutter/clutter-paint-nodes.c

@@ -1338,6 +1338,8 @@ struct _ClutterLayerNode
   CoglFramebuffer *offscreen;
 
   guint8 opacity;
+
+  gboolean needs_fbo_setup : 1;
 };
 
 struct _ClutterLayerNodeClass
@@ -1365,25 +1367,27 @@ clutter_layer_node_pre_draw (ClutterPaintNode *node,
   if (node->operations == NULL)
     return FALSE;
 
-  /* copy the same modelview from the current framebuffer to the one we
+  if (lnode->needs_fbo_setup)
-   * are going to use
+    {
-   */
+      /* copy the same modelview from the current framebuffer to the one we
-  framebuffer = clutter_paint_context_get_framebuffer (paint_context);
+       * are going to use
-  cogl_framebuffer_get_modelview_matrix (framebuffer, &matrix);
+       */
+      framebuffer = clutter_paint_context_get_framebuffer (paint_context);
+      cogl_framebuffer_get_modelview_matrix (framebuffer, &matrix);
+      cogl_framebuffer_set_modelview_matrix (lnode->offscreen, &matrix);
+
+      cogl_framebuffer_set_viewport (lnode->offscreen,
+                                     lnode->viewport.x,
+                                     lnode->viewport.y,
+                                     lnode->viewport.width,
+                                     lnode->viewport.height);
+
+      cogl_framebuffer_set_projection_matrix (lnode->offscreen,
+                                              &lnode->projection);
+    }
 
   clutter_paint_context_push_framebuffer (paint_context, lnode->offscreen);
 
-  cogl_framebuffer_set_modelview_matrix (lnode->offscreen, &matrix);
-
-  cogl_framebuffer_set_viewport (lnode->offscreen,
-                                 lnode->viewport.x,
-                                 lnode->viewport.y,
-                                 lnode->viewport.width,
-                                 lnode->viewport.height);
-
-  cogl_framebuffer_set_projection_matrix (lnode->offscreen,
-                                          &lnode->projection);
-
   /* clear out the target framebuffer */
   cogl_framebuffer_clear4f (lnode->offscreen,
                             COGL_BUFFER_BIT_COLOR | COGL_BUFFER_BIT_DEPTH,
@@ -1522,6 +1526,7 @@ clutter_layer_node_new (const graphene_matrix_t *projection,
 
   res = _clutter_paint_node_create (CLUTTER_TYPE_LAYER_NODE);
 
+  res->needs_fbo_setup = TRUE;
   res->projection = *projection;
   res->viewport = *viewport;
   res->fbo_width = width;
@@ -1566,3 +1571,39 @@ out:
 
   return (ClutterPaintNode *) res;
 }
+
+/**
+ * clutter_layer_node_new_to_framebuffer:
+ * @framebuffer: a #CoglFramebuffer
+ * @pipeline: a #CoglPipeline
+ *
+ * Creates a new #ClutterLayerNode that will redirect drawing at
+ * @framebuffer. It will then use @pipeline to paint the stored
+ * operations.
+ *
+ * When using this constructor, the caller is reponsible for setting
+ * up @framebuffer, including its modelview and projection matrices,
+ * and the viewport, and the @pipeline as well.
+ *
+ * Return value: (transfer full): the newly created #ClutterLayerNode.
+ *   Use clutter_paint_node_unref() when done.
+ */
+ClutterPaintNode *
+clutter_layer_node_new_to_framebuffer (CoglFramebuffer *framebuffer,
+                                       CoglPipeline    *pipeline)
+{
+  ClutterLayerNode *res;
+
+  g_return_val_if_fail (COGL_IS_FRAMEBUFFER (framebuffer), NULL);
+  g_return_val_if_fail (cogl_is_pipeline (pipeline), NULL);
+
+  res = _clutter_paint_node_create (CLUTTER_TYPE_LAYER_NODE);
+
+  res->needs_fbo_setup = FALSE;
+  res->fbo_width = cogl_framebuffer_get_width (framebuffer);
+  res->fbo_height = cogl_framebuffer_get_height (framebuffer);
+  res->offscreen = g_object_ref (framebuffer);
+  res->pipeline = cogl_pipeline_copy (pipeline);
+
+  return (ClutterPaintNode *) res;
+}

clutter/clutter/clutter-paint-nodes.h

@@ -209,6 +209,10 @@ ClutterPaintNode *      clutter_layer_node_new          (const graphene_matrix_t
                                                          float                    height,
                                                          guint8                   opacity);
 
+CLUTTER_EXPORT
+ClutterPaintNode * clutter_layer_node_new_to_framebuffer (CoglFramebuffer *framebuffer,
+                                                          CoglPipeline    *pipeline);
+
 
 #define CLUTTER_TYPE_TRANSFORM_NODE             (clutter_transform_node_get_type ())
 #define CLUTTER_TRANSFORM_NODE(obj)             (G_TYPE_CHECK_INSTANCE_CAST ((obj), CLUTTER_TYPE_TRANSFORM_NODE, ClutterTransformNode))