[raudio] Implement GetMasterVolume() (#3434)

It feels a little unfinished when you can SetMasterVolume but can't
really Get it. So to finish the symmetry here is the GetMasterVolume
implementation.

src/raudio.c

@@ -536,6 +536,14 @@ void SetMasterVolume(float volume)
     ma_device_set_master_volume(&AUDIO.System.device, volume);
 }
 
+// Get master volume (listener)
+float GetMasterVolume(void)
+{
+    float volume = 0.0f;
+    ma_device_get_master_volume(&AUDIO.System.device, &volume);
+    return volume;
+}
+
 //----------------------------------------------------------------------------------
 // Module Functions Definition - Audio Buffer management
 //----------------------------------------------------------------------------------

src/raylib.h

@@ -1538,6 +1538,7 @@ RLAPI void InitAudioDevice(void);                                     // Initial
 RLAPI void CloseAudioDevice(void);                                    // Close the audio device and context
 RLAPI bool IsAudioDeviceReady(void);                                  // Check if audio device has been initialized successfully
 RLAPI void SetMasterVolume(float volume);                             // Set master volume (listener)
+RLAPI float GetMasterVolume(void);                                    // Get master volume (listener)
 
 // Wave/Sound loading/unloading functions
 RLAPI Wave LoadWave(const char *fileName);                            // Load wave data from file