libobs: Add volume meter object

This adds a volume meter object to libobs that can be used by the GUI
or plugins to convert the raw audio level data from sources to values
that can easily be used to display the audio data.
The volume meter object will use the same mapping functions as the
fader object to map dB levels to a scale.

libobs/obs-audio-controls.h

@@ -137,6 +137,53 @@ EXPORT void obs_fader_detach_source(obs_fader_t *fader);
  */
 EXPORT signal_handler_t *obs_fader_get_signal_handler(obs_fader_t *fader);
 
+/**
+ * @brief Create a volume meter
+ * @param type the mapping type to use for the volume meter
+ * @return pointer to the volume meter object
+ *
+ * A volume meter object is used to prepare the sound levels reported by audio
+ * sources for display in a GUI.
+ * It will automatically take source volume into account and map the levels
+ * to a range [0.0f, 1.0f].
+ */
+EXPORT obs_volmeter_t *obs_volmeter_create(enum obs_fader_type type);
+
+/**
+ * @brief Destroy a volume meter
+ * @param volmeter pointer to the volmeter object
+ *
+ * Destroy the volume meter and free all related data
+ */
+EXPORT void obs_volmeter_destroy(obs_volmeter_t *volmeter);
+
+/**
+ * @brief Attach the volume meter to a source
+ * @param volmeter pointer to the volume meter object
+ * @param source pointer to the source object
+ * @return true on success
+ *
+ * When the volume meter is attached to a source it will start to listen to
+ * volume updates on the source and after preparing the data emit its own
+ * signal.
+ */
+EXPORT bool obs_volmeter_attach_source(obs_volmeter_t *volmeter,
+		obs_source_t *source);
+
+/**
+ * @brief Detach the volume meter from the currently attached source
+ * @param volmeter pointer to the volume meter object
+ */
+EXPORT void obs_volmeter_detach_source(obs_volmeter_t *volmeter);
+
+/**
+ * @brief Get signal handler for the volume meter object
+ * @param volmeter pointer to the volume meter object
+ * @return signal handler
+ */
+EXPORT signal_handler_t *obs_volmeter_get_signal_handler(
+		obs_volmeter_t *volmeter);
+
 #ifdef __cplusplus
 }
 #endif