Piano: Quit the audio thread by checking Core::EventLoop for exit

The infinite loop here doesn't really work at all for an application
process that expects to be able to exit. Check against
Core::EventLoop::current() to see if it's time to
exit, and return 0 from the thread function if so.

The thread will be joined in its destructor, which doesn't assert
anymore now that Thread is a jthread.

Applications/Piano/main.cpp

@@ -82,7 +82,7 @@ int main(int argc, char** argv)
         }
 
         Array<Sample, sample_count> buffer;
-        for (;;) {
+        while (!Core::EventLoop::current().was_exit_requested()) {
             track_manager.fill_buffer(buffer);
             audio->write(reinterpret_cast<u8*>(buffer.data()), buffer_size);
             Core::EventLoop::current().post_event(main_widget, make<Core::CustomEvent>(0));
@@ -101,6 +101,7 @@ int main(int argc, char** argv)
                 wav_writer.finalize();
             }
         }
+        return 0;
     });
     audio_thread->start();