decoder: automatically flush the output buffer after decoder exits

A decoder_flush() invocation was missing in the FLAC plugin, resulting
in casual assertion failures due to a wrong assumption about the last
chunk's audio format.  It's much easier to remove that decoder_flush()
function and make the decoder thread call ob_flush().

src/decoder_thread.c

@@ -21,6 +21,7 @@
 #include "decoder_control.h"
 #include "decoder_internal.h"
 #include "player_control.h"
+#include "outputBuffer.h"
 #include "song.h"
 #include "mapper.h"
 #include "path.h"
@@ -147,6 +148,8 @@ static void decodeStart(void)
 		}
 	}
 
+	ob_flush();
+
 	if (ret < 0 || ret == DECODE_ERROR_UNKTYPE) {
 		if (ret != DECODE_ERROR_UNKTYPE)
 			dc.error = DECODE_ERROR_FILE;