meta: Add a new "clear" transform to avoid re-allocations

In the buffer pool, try to clear metas before freeing them so we
avoid constant reallocations on every frame.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4962>

subprojects/gstreamer/gst/gstbufferpool.c

@@ -1217,6 +1217,14 @@ remove_meta_unpooled (GstBuffer * buffer, GstMeta ** meta, gpointer user_data)
   if (!GST_META_FLAG_IS_SET (*meta, GST_META_FLAG_POOLED)) {
     GST_META_FLAG_UNSET (*meta, GST_META_FLAG_LOCKED);
     *meta = NULL;
+  } else {
+    const GstMetaInfo *info = (*meta)->info;
+
+    /* If we can clear it, don't free it */
+    if (info->transform_func) {
+      info->transform_func (NULL, *meta, buffer, _gst_meta_transform_clear,
+          NULL);
+    }
   }
   return TRUE;
 }

subprojects/gstreamer/gst/gstmeta.c

@@ -56,6 +56,7 @@ static GHashTable *metainfo = NULL;
 static GRWLock lock;
 
 GQuark _gst_meta_transform_copy;
+GQuark _gst_meta_transform_clear;
 GQuark _gst_meta_tag_memory;
 GQuark _gst_meta_tag_memory_reference;
 
@@ -81,6 +82,7 @@ _priv_gst_meta_initialize (void)
   metainfo = g_hash_table_new_full (g_str_hash, g_str_equal, NULL, free_info);
 
   _gst_meta_transform_copy = g_quark_from_static_string ("gst-copy");
+  _gst_meta_transform_clear = g_quark_from_static_string ("gst-clear");
   _gst_meta_tag_memory = g_quark_from_static_string ("memory");
   _gst_meta_tag_memory_reference =
       g_quark_from_static_string ("memory-reference");

subprojects/gstreamer/gst/gstmeta.h

@@ -80,6 +80,7 @@ typedef enum {
  * @flag: the #GstMetaFlags to clear.
  *
  * Clears a metadata flag.
+ *
  */
 #define GST_META_FLAG_UNSET(meta,flag)         (GST_META_FLAGS (meta) &= ~(flag))
 
@@ -189,6 +190,28 @@ typedef struct {
   gsize size;
 } GstMetaTransformCopy;
 
+/**
+ * gst_meta_transform_clear:
+ *
+ * GQuark for the "gst-clear" transform.
+ *
+ * Since: 1.24
+ */
+
+GST_API GQuark _gst_meta_transform_clear;
+
+/**
+ * GST_META_TRANSFORM_IS_CLEAR:
+ * @type: a transform type
+ *
+ * Check if the transform type is clearing the content of the meta without
+ * freeing it.
+ *
+ * Since: 1.24
+ */
+#define GST_META_TRANSFORM_IS_CLEAR(type) ((type) == _gst_meta_transform_clear)
+
+
 /**
  * GstMetaTransformFunction:
  * @transbuf: a #GstBuffer