filter/Volume: add volume_filter_prepare()

Circumvent filter_new() with a fake ConfigBlock.

src/filter/plugins/VolumeFilterPlugin.cxx

@@ -78,6 +78,12 @@ const FilterPlugin volume_filter_plugin = {
 	volume_filter_init,
 };
 
+PreparedFilter *
+volume_filter_prepare() noexcept
+{
+	return new PreparedVolumeFilter();
+}
+
 unsigned
 volume_filter_get(const Filter *_filter) noexcept
 {

src/filter/plugins/VolumeFilterPlugin.hxx

@@ -20,8 +20,12 @@
 #ifndef MPD_VOLUME_FILTER_PLUGIN_HXX
 #define MPD_VOLUME_FILTER_PLUGIN_HXX
 
+class PreparedFilter;
 class Filter;
 
+PreparedFilter *
+volume_filter_prepare() noexcept;
+
 unsigned
 volume_filter_get(const Filter *filter) noexcept;
 

src/output/Init.cxx

@@ -33,6 +33,7 @@
 #include "filter/plugins/AutoConvertFilterPlugin.hxx"
 #include "filter/plugins/ReplayGainFilterPlugin.hxx"
 #include "filter/plugins/ChainFilterPlugin.hxx"
+#include "filter/plugins/VolumeFilterPlugin.hxx"
 #include "config/ConfigError.hxx"
 #include "config/ConfigGlobal.hxx"
 #include "config/Block.hxx"
@@ -106,12 +107,6 @@ audio_output_mixer_type(const ConfigBlock &block) noexcept
 						  "hardware"));
 }
 
-static PreparedFilter *
-CreateVolumeFilter()
-{
-	return filter_new(&volume_filter_plugin, ConfigBlock());
-}
-
 static Mixer *
 audio_output_load_mixer(EventLoop &event_loop, AudioOutput &ao,
 			const ConfigBlock &block,
@@ -144,7 +139,7 @@ audio_output_load_mixer(EventLoop &event_loop, AudioOutput &ao,
 		assert(mixer != nullptr);
 
 		filter_chain_append(filter_chain, "software_mixer",
-				    ao.volume_filter.Set(CreateVolumeFilter()));
+				    ao.volume_filter.Set(volume_filter_prepare()));
 		return mixer;
 	}