[sclang][win] read stdin sync before starting async service

this fixes supercollider#4214 - the initial read of stdin was being lost while starting the async reading service

lang/LangSource/SC_TerminalClient.cpp

@@ -590,6 +590,17 @@ void SC_TerminalClient::inputThreadFn() {
         readlineInit();
 #endif
 
+#ifdef _WIN32
+    // make sure there's nothing on stdin before we launch the service
+    // this fixes #4214
+    DWORD bytesRead = 0;
+    auto success = ReadFile(GetStdHandle(STD_INPUT_HANDLE), inputBuffer.data(), inputBuffer.size(), &bytesRead, NULL);
+
+    if (success) {
+        pushCmdLine(inputBuffer.data(), bytesRead);
+    }
+#endif
+
     startInputRead();
 
     boost::asio::io_service::work work(mInputService);