SoundPlayer: Introduce buffer_reset api to VisualizationWidget

Previously there was no way to clear the internal buffer in a
VisualizationWidget. These buffers are what determines how a
VisualizationWidget is rendered, so with this commit we are now
able to reset visualizations.

Userland/Applications/SoundPlayer/VisualizationWidget.h

@@ -33,6 +33,13 @@ public:
         m_frame_count = 0;
     }
 
+    void reset_buffer()
+    {
+        m_sample_buffer.clear();
+        m_render_buffer.fill_with(.0f);
+        m_frame_count = 0;
+    }
+
     virtual void set_samplerate(int samplerate)
     {
         m_samplerate = samplerate;