pcm/Export: add (dummy) method Cancel()

We'll have some code for it soon.

src/output/plugins/AlsaOutputPlugin.cxx

@@ -826,6 +826,8 @@ AlsaOutput::Cancel()
 	must_prepare = true;
 
 	snd_pcm_drop(pcm);
+
+	pcm_export->Reset();
 }
 
 inline void

src/output/plugins/OssOutputPlugin.cxx

@@ -659,6 +659,10 @@ OssOutput::Cancel()
 		ioctl(fd, SNDCTL_DSP_RESET, 0);
 		DoClose();
 	}
+
+#ifdef AFMT_S24_PACKED
+	pcm_export->Reset();
+#endif
 }
 
 inline size_t

src/pcm/PcmExport.hxx

@@ -149,6 +149,12 @@ struct PcmExport {
 	void Open(SampleFormat sample_format, unsigned channels,
 		  Params params);
 
+	/**
+	 * Reset the filter's state, e.g. drop/flush buffers.
+	 */
+	void Reset() {
+	}
+
 	/**
 	 * Calculate the size of one output frame.
 	 */