DecoderAPI: add function decoder_open_uri()

Move and refactor code from the Wavpack decoder plugin.

test/FakeDecoderAPI.cxx

@@ -66,6 +66,12 @@ decoder_seek_error(gcc_unused Decoder &decoder)
 {
 }
 
+InputStream *
+decoder_open_uri(Decoder &decoder, const char *uri, Error &error)
+{
+	return InputStream::OpenReady(uri, decoder.mutex, decoder.cond, error);
+}
+
 size_t
 decoder_read(gcc_unused Decoder *decoder,
 	     InputStream &is,

test/FakeDecoderAPI.hxx

@@ -21,8 +21,13 @@
 #define FAKE_DECODER_API_HXX
 
 #include "check.h"
+#include "thread/Mutex.hxx"
+#include "thread/Cond.hxx"
 
 struct Decoder {
+	Mutex mutex;
+	Cond cond;
+
 	bool initialized;
 
 	Decoder()

test/run_decoder.cxx

@@ -27,7 +27,6 @@
 #include "fs/Path.hxx"
 #include "AudioFormat.hxx"
 #include "util/Error.hxx"
-#include "thread/Cond.hxx"
 #include "Log.hxx"
 #include "stdbin.h"
 
@@ -77,11 +76,9 @@ int main(int argc, char **argv)
 	if (plugin->file_decode != nullptr) {
 		plugin->FileDecode(decoder, Path::FromFS(uri));
 	} else if (plugin->stream_decode != nullptr) {
-		Mutex mutex;
-		Cond cond;
-
 		InputStream *is =
-			InputStream::OpenReady(uri, mutex, cond, error);
+			InputStream::OpenReady(uri, decoder.mutex,
+					       decoder.cond, error);
 		if (is == NULL) {
 			if (error.IsDefined())
 				LogError(error);