output: attach a filter chain to each audio_output

This patch adds initial filter support for audio outputs.  Each audio
output gets a "filter" attribute, which is used by ao_play_chunk().

The PCM conversion is now performed by convert_filter_plugin.
audio_output.convert_state has been removed.

test/run_output.c

@@ -22,6 +22,8 @@
 #include "output_control.h"
 #include "conf.h"
 #include "audio_parser.h"
+#include "filter_registry.h"
+#include "pcm_convert.h"
 
 #include <glib.h>
 
@@ -33,10 +35,31 @@ void pcm_convert_init(G_GNUC_UNUSED struct pcm_convert_state *state)
 {
 }
 
+void pcm_convert_deinit(G_GNUC_UNUSED struct pcm_convert_state *state)
+{
+}
+
+const void *
+pcm_convert(G_GNUC_UNUSED struct pcm_convert_state *state,
+	    G_GNUC_UNUSED const struct audio_format *src_format,
+	    G_GNUC_UNUSED const void *src, G_GNUC_UNUSED size_t src_size,
+	    G_GNUC_UNUSED const struct audio_format *dest_format,
+	    G_GNUC_UNUSED size_t *dest_size_r)
+{
+	return NULL;
+}
+
 void notify_init(G_GNUC_UNUSED struct notify *notify)
 {
 }
 
+const struct filter_plugin *
+filter_plugin_by_name(G_GNUC_UNUSED const char *name)
+{
+	assert(false);
+	return NULL;
+}
+
 static const struct config_param *
 find_named_config_block(const char *block, const char *name)
 {