pcm/PcmConvert: eliminate Open() and Close()

Let the constructor and destructor do this.  This means that all users
have to be converted to allocate PcmConvert dynamically.

test/RunChromaprint.cxx

@@ -18,6 +18,7 @@
  */
 
 #include "ConfigGlue.hxx"
+#include "pcm/PcmConvert.hxx"
 #include "lib/chromaprint/DecoderClient.hxx"
 #include "event/Thread.hxx"
 #include "decoder/DecoderList.hxx"

test/run_convert.cxx

@@ -53,8 +53,7 @@ try {
 
 	const size_t in_frame_size = in_audio_format.GetFrameSize();
 
-	PcmConvert state;
-	state.Open(in_audio_format, out_audio_format);
+	PcmConvert state(in_audio_format, out_audio_format);
 
 	StaticFifoBuffer<uint8_t, 4096> buffer;
 
@@ -94,8 +93,6 @@ try {
 						   output.size);
 	}
 
-	state.Close();
-
 	return EXIT_SUCCESS;
 } catch (...) {
 	PrintException(std::current_exception());