buffer: add gst_buffer_iterate_meta_filtered()

For convenience. Pretty much every user of
gst_buffer_iterate_meta() filters for a specific
api type.

https://bugzilla.gnome.org/show_bug.cgi?id=775727

docs/gst/gstreamer-sections.txt

@@ -254,6 +254,7 @@ gst_buffer_get_meta
 gst_buffer_add_meta
 gst_buffer_remove_meta
 gst_buffer_iterate_meta
+gst_buffer_iterate_meta_filtered
 GstBufferForeachMetaFunc
 gst_buffer_foreach_meta
 

gst/gstbuffer.c

@@ -2276,6 +2276,49 @@ gst_buffer_iterate_meta (GstBuffer * buffer, gpointer * state)
     return NULL;
 }
 
+/**
+ * gst_buffer_iterate_meta_filtered: (skip)
+ * @buffer: a #GstBuffer
+ * @state: an opaque state pointer
+ * @meta_api_type: only return #GstMeta of this type
+ *
+ * Retrieve the next #GstMeta of type @meta_api_type after the current one
+ * according to @state. If @state points to %NULL, the first metadata of
+ * type @meta_api_type is returned.
+ *
+ * @state will be updated with an opaque state pointer
+ *
+ * Returns: (transfer none) (nullable): The next #GstMeta of type
+ * @meta_api_type or %NULL when there are no more items.
+ *
+ * Since: 1.12
+ */
+GstMeta *
+gst_buffer_iterate_meta_filtered (GstBuffer * buffer, gpointer * state,
+    GType meta_api_type)
+{
+  GstMetaItem **meta;
+
+  g_return_val_if_fail (buffer != NULL, NULL);
+  g_return_val_if_fail (state != NULL, NULL);
+
+  meta = (GstMetaItem **) state;
+  if (*meta == NULL)
+    /* state NULL, move to first item */
+    *meta = GST_BUFFER_META (buffer);
+  else
+    /* state !NULL, move to next item in list */
+    *meta = (*meta)->next;
+
+  while (*meta != NULL && (*meta)->meta.info->api != meta_api_type)
+    *meta = (*meta)->next;
+
+  if (*meta)
+    return &(*meta)->meta;
+  else
+    return NULL;
+}
+
 /**
  * gst_buffer_foreach_meta:
  * @buffer: a #GstBuffer

gst/gstbuffer.h

@@ -545,6 +545,10 @@ gboolean        gst_buffer_remove_meta          (GstBuffer *buffer, GstMeta *met
 
 GstMeta *       gst_buffer_iterate_meta         (GstBuffer *buffer, gpointer *state);
 
+GstMeta *       gst_buffer_iterate_meta_filtered (GstBuffer * buffer,
+                                                  gpointer  * state,
+                                                  GType       meta_api_type);
+
 gboolean        gst_buffer_foreach_meta         (GstBuffer *buffer,
                                                  GstBufferForeachMetaFunc func,
                                                  gpointer user_data);

win32/common/libgstreamer.def

@@ -144,6 +144,7 @@ EXPORTS
 	gst_buffer_is_all_memory_writable
 	gst_buffer_is_memory_range_writable
 	gst_buffer_iterate_meta
+	gst_buffer_iterate_meta_filtered
 	gst_buffer_list_copy_deep
 	gst_buffer_list_foreach
 	gst_buffer_list_get