videoaggregator: Move needs_alpha pad field to the private struct

And also trigger renegotiation if the value has changed.

https://bugzilla.gnome.org/show_bug.cgi?id=795836

gst-libs/gst/video/gstvideoaggregator.c

@@ -72,6 +72,10 @@ struct _GstVideoAggregatorPadPrivate
   guint zorder;
   gboolean repeat_after_eos;
 
+  /* Subclasses can force an alpha channel in the (input thus output)
+   * colorspace format */
+  gboolean needs_alpha;
+
   /* Converter, if NULL no conversion is done */
   GstVideoConverter *convert;
 
@@ -435,6 +439,32 @@ gst_video_aggregator_pad_get_prepared_frame (GstVideoAggregatorPad * pad)
   return pad->priv->prepared_frame.buffer ? &pad->priv->prepared_frame : NULL;
 }
 
+/**
+ * gst_video_aggregator_pad_set_needs_alpha:
+ * @pad: a #GstVideoAggregatorPad
+ * @needs_alpha: %TRUE if this pad requires alpha output
+ *
+ * Allows selecting that this pad requires an output format with alpha
+ *
+ * Returns: (transfer none): The currently prepared video frame
+ */
+void
+gst_video_aggregator_pad_set_needs_alpha (GstVideoAggregatorPad * pad,
+    gboolean needs_alpha)
+{
+  g_return_if_fail (GST_IS_VIDEO_AGGREGATOR_PAD (pad));
+
+  if (needs_alpha != pad->priv->needs_alpha) {
+    GstAggregator *agg =
+        GST_AGGREGATOR (gst_object_get_parent (GST_OBJECT (pad)));
+    pad->priv->needs_alpha = needs_alpha;
+    if (agg) {
+      gst_pad_mark_reconfigure (GST_AGGREGATOR_SRC_PAD (agg));
+      gst_object_unref (agg);
+    }
+  }
+}
+
 /**************************************
  * GstVideoAggregator implementation  *
  **************************************/
@@ -562,7 +592,7 @@ gst_video_aggregator_find_best_format (GstVideoAggregator * vagg,
         GINT_TO_POINTER (format_number));
 
     /* If that pad is the first with alpha, set it as the new best format */
-    if (!need_alpha && (pad->needs_alpha
+    if (!need_alpha && (pad->priv->needs_alpha
             && (!GST_VIDEO_FORMAT_INFO_HAS_ALPHA (pad->info.finfo)))) {
       need_alpha = TRUE;
       /* Just fallback to ARGB in case we require alpha but the input pad

gst-libs/gst/video/gstvideoaggregator.h

@@ -71,10 +71,6 @@ struct _GstVideoAggregatorPad
   /* read-only, with OBJECT_LOCK */
   GstVideoInfo info;
 
-  /* Subclasses can force an alpha channel in the (input thus output)
-   * colorspace format */
-  gboolean needs_alpha;
-
   /* < private > */
   GstVideoAggregatorPadPrivate *priv;
 
@@ -122,6 +118,9 @@ GstBuffer * gst_video_aggregator_pad_get_current_buffer (GstVideoAggregatorPad *
 GST_VIDEO_BAD_API
 GstVideoFrame * gst_video_aggregator_pad_get_prepared_frame (GstVideoAggregatorPad *pad);
 
+GST_VIDEO_BAD_API
+void gst_video_aggregator_pad_set_needs_alpha (GstVideoAggregatorPad *pad, gboolean needs_alpha);
+
 #define GST_TYPE_VIDEO_AGGREGATOR (gst_video_aggregator_get_type())
 #define GST_VIDEO_AGGREGATOR(obj) \
         (G_TYPE_CHECK_INSTANCE_CAST((obj),GST_TYPE_VIDEO_AGGREGATOR, GstVideoAggregator))

gst/compositor/compositor.c

@@ -198,7 +198,8 @@ gst_compositor_pad_set_property (GObject * object, guint prop_id,
       break;
     case PROP_PAD_CROSSFADE_RATIO:
       pad->crossfade = g_value_get_double (value);
-      GST_VIDEO_AGGREGATOR_PAD (pad)->needs_alpha = pad->crossfade >= 0.0f;
+      gst_video_aggregator_pad_set_needs_alpha (GST_VIDEO_AGGREGATOR_PAD (pad),
+          pad->crossfade >= 0.0f);
       break;
     default:
       G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);