FilterInternal: convert struct filter to a OO interface

src/mixer/SoftwareMixerPlugin.cxx

@@ -32,7 +32,7 @@ struct software_mixer {
 	/** the base mixer class */
 	struct mixer base;
 
-	struct filter *filter;
+	Filter *filter;
 
 	unsigned volume;
 };
@@ -100,7 +100,7 @@ const struct mixer_plugin software_mixer_plugin = {
 	true,
 };
 
-struct filter *
+Filter *
 software_mixer_get_filter(struct mixer *mixer)
 {
 	struct software_mixer *sm = (struct software_mixer *)mixer;

src/mixer/SoftwareMixerPlugin.hxx

@@ -21,13 +21,13 @@
 #define MPD_SOFTWARE_MIXER_PLUGIN_HXX
 
 struct mixer;
-struct filter;
+class Filter;
 
 /**
  * Returns the (volume) filter associated with this mixer.  All users
  * of this mixer plugin should install this filter.
  */
-struct filter *
+Filter *
 software_mixer_get_filter(struct mixer *mixer);
 
 #endif