Implement param filtering

Make a new pod filter function and use it in the plugins to filter
in enum_params.
Small tweaks to the pod_builder

spa/lib/pod.h

@@ -0,0 +1,41 @@
+/* Simple Plugin API
+ * Copyright (C) 2017 Wim Taymans <wim.taymans@gmail.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Library General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Library General Public License for more details.
+ *
+ * You should have received a copy of the GNU Library General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 51 Franklin St, Fifth Floor,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#ifndef __SPA_LIBPOD_H__
+#define __SPA_LIBPOD_H__
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include <spa/props.h>
+#include <spa/pod-builder.h>
+
+int spa_pod_filter(struct spa_pod_builder *b,
+		   const struct spa_pod *pod,
+		   const struct spa_pod *filter);
+
+int spa_pod_compare(const struct spa_pod *pod1,
+		    const struct spa_pod *pod2);
+
+#ifdef __cplusplus
+} /* extern "C" */
+#endif
+
+#endif /* __SPA_LIBPOD_H__ */