basetransform: Add a method to let subclasses cleanly update srcpad caps

API:
    gst_base_transform_update_src

https://bugzilla.gnome.org/show_bug.cgi?id=734424

docs/libs/gstreamer-libs-sections.txt

@@ -361,6 +361,7 @@ gst_base_transform_get_allocator
 gst_base_transform_get_buffer_pool
 gst_base_transform_reconfigure_sink
 gst_base_transform_reconfigure_src
+gst_base_transform_update_src
 
 GST_BASE_TRANSFORM_SINK_NAME
 GST_BASE_TRANSFORM_SRC_NAME

libs/gst/base/gstbasetransform.c

@@ -2786,3 +2786,33 @@ gst_base_transform_get_allocator (GstBaseTransform * trans,
   if (params)
     *params = trans->priv->params;
 }
+
+/**
+ * gst_base_transform_update_src_caps:
+ * @trans: a #GstBaseTransform
+ * @updated_caps: An updated version of the srcpad caps to be pushed
+ * downstream
+ *
+ * Updates the srcpad caps and send the caps downstream. This function
+ * can be used by subclasses when they have already negotiated their caps
+ * but found a change in them (or computed new informations). This way,
+ * they can notify downstream about that change without loosing any
+ * buffer.
+ *
+ * Returns: %TRUE if the caps could be send downstream %FALSE otherwise
+ */
+gboolean
+gst_base_transform_update_src_caps (GstBaseTransform * trans,
+    GstCaps * updated_caps)
+{
+  g_return_val_if_fail (GST_IS_BASE_TRANSFORM (trans), FALSE);
+
+  if (gst_pad_push_event (GST_BASE_TRANSFORM_SRC_PAD (trans),
+          gst_event_new_caps (updated_caps))) {
+    gst_pad_mark_reconfigure (trans->srcpad);
+
+    return TRUE;
+  }
+
+  return FALSE;
+}

libs/gst/base/gstbasetransform.h

@@ -293,6 +293,8 @@ void            gst_base_transform_get_allocator    (GstBaseTransform *trans,
 
 void		gst_base_transform_reconfigure_sink (GstBaseTransform *trans);
 void		gst_base_transform_reconfigure_src  (GstBaseTransform *trans);
+gboolean gst_base_transform_update_src_caps (GstBaseTransform *trans,
+                                             GstCaps *updated_caps);
 G_END_DECLS
 
 #endif /* __GST_BASE_TRANSFORM_H__ */